When i start my system bios error is appearing

Lenovo Basic Input/Output System (BIOS) settings allow you to quickly customize interactions between your operating system and your hardware, especially your hard drive or drives, memory and power supply.
You must get into the Lenovo BIOS before your operating system loads.
Though most users don't typically see options for entering or using the BIOS, multiple changes can be made to customize and optimize your system, such as changing your boot device order and changing the display brightness to conserve power.

Turn off your Lenovo computer.
Press the power button to start your system.
For best results, wait 30 seconds to one minute before starting your system.
This gives you a longer window in which to enter the BIOS.

Press "F1" as soon as the Thinkpad logo appears.
Depending on your Thinkpad model, you may be prompted to press either the blue "Access IBM" or the blue "ThinkAdvantage" button instead of "F1 or F2."
If this is the case, a message will appear stating "To interrupt normal startup, press (one of the listed buttons above)."
Select the "Start setup utility" to open the BIOS setup menu.
Follow the instructions provided in the BIOS setup menu to select BIOS menu sections, settings and to change options.
Next, navigate through the Bios setup pages using keybard arrow keys in order to find failsafe or optimized defaults press ENTER to select your option press F10 save to cmos and restart

http://www.michaelstevenstech.com/lenovoBIOS.html

Reboot windows without any CD in drive and tap F8 key before windows screen appears. Select safe mode from the menu that appears and elect to do a system restore.
If this doesn't work you could try changing the boot order in the bios ensuring that your hard drive is first boot device. should get into bios when starting PC usually F2 or del key. remember to save changes in bios normally F10

2000-0142 and 2000-0146 are hard drive SMART errors, indicating the HDD will fail.
To boot message about the AC adapter was a BIOS level message that normally appears when a Auto/air ac adapter or other 65 watt ac adapter is used to power the system.

This "0251" error indicates that the CMOS (complementary metal-oxide semiconductor; a memory chip that holds data the computer needs each time it starts, such as the current time and the BIOS [Basic Input/Output System]) has become corrupted or damaged, typically because of one of these three reasons: A BIOS update has gone awry, a recently installed program incorrectly modified the BIOS, or the computer battery is running out of power.

If you recently upgraded the BIOS, the first thing you should do is edit the BIOS settings. The BIOS developer can help you access the BIOS Setup utility and configure the BIOS accordingly. Likewise, if you recently installed a program that may have modified the BIOS, contact the software developer for assistance in reconfiguring the BIOS for the new program.
If you did neither of these things, the culprit is probably a faltering battery. You can probably replace the battery on your own, but the process varies widely depending on the type and age of the computer. We suggest consulting the documentation that came with your computer or contact the computer manufacturer to determine the type of battery involved, where to purchase a new one, and how to replace it. You can also access the BIOS Setup utility and write down the BIOS settings to reconfigure the BIOS correctly after installing the replacement battery. Once the new battery is in place, you will have to reconfigure the BIOS and reset the system time, but the error message shouldn't appear again.

hope this helps

did u open ur bios and set it to configure w/ ur system?
Removing the battery means reseting ur BiOS!

If you receive the message "Error loading operating system" and before Windows was running normally, you can try to reinstall your OS with the repair option (if you can't access your hard drive then it's hdd failure). If your message appears before boot media is accessed, I would recommend you to reset BIOS settings by following the steps below:

Hello my friend.That error message has nothing to do with the actual keyboard.The POST/BIOS is reading a single port bit in the system boards keyboard controller and detecting this bit as being shorted to ground. On some systems there is a keylock on the front of the case that allows you to "lock" the keyboard for security reasons. It is this lock that is being detected. On a system without a keylock, this error can be caused by one of the front panel wires being placed on the wrong pin on the system board. Or by something shorting out the keylock pin on the system board, or a hardware failure in the keyboard controller, or using the wrong version of BIOS.So make sure if your computer has a lock key/button or something on the case that locks the keyboard.Also you can load default bios settings by hitting Del while you boot if you have change anything in the bios.

The 'Press F1 to continue" message is a result of an error caused as the BIOS does its hardware checks during the boot process.

The BIOS boot process (bootstrap) preceeds the loading of the operating system.

The most amusing example of this occurs if you unplug a ps2 keyboard, 
1, it generates the error
2, pressing F1 does no good because the keyboard is not plugged in.
3, if its ps2, you will need to do a hardware reset after plugging the keyboard in again.

The bios can be set to 'halt on'
'no errors' which will stop the error message appearing.
'keyboard errors' see above
'all errors' any hardware failure detected by the bios will halt the boot process and give the above message.

The System Event Log may become full of erroneous IPMI Events reported by the System BIOS. When this occurs, the following message may appear during Power-On Self-Test (POST) and POST cannot continue without user input:
02a2: BMC System Error Log (SEL) Full
Press F1 to Continue, Press F2 to Setup
If the above message appears during POST, perform the following steps to run BIOS setup and delete the System Event Log:

1. Boot the server and press the F10 key when prompted to run BIOS Setup.
2. Select the "Advanced" menu.
3. Scroll down to "IPMI" and press the ENTER key
4. Scroll down to the System Event Log and press the ENTER key.
5. At the "Clear System Event Log" selection, press the ENTER key to toggle between "Enable" and "Disable."
6. Select "Enable."
7. Press the F10 key to Save the changes and exit the BIOS Setup.

The Setup option is for your BIOS , heres a link to resetting the BIOS password if you want to access it.

but as for the error loading operating system you will need any setup disks or recovery CD's that you got with your computer, preferably a Windows Installation CD , then you will have to insert the CD restart and let the computer boot from the CD and reinstall your operating system.