Lenovo Ideacentre A300 bios - Lenovo 40181EU IdeaCentre A300 AllInOne PC PC Desktop

Here is a YouTube video showing how to replace the CPU on an IdeaCentre A310, which should be similar to the A300. When he takes off the back cover, you can see the hard drive on the right side.
CPULenovo IdeaCentre A310 Core i3 350MCore i5 460M

Are you trying to enter (get into) BIOS?

This is for another model of Lenovo on the Lenovo.com website, but it will likely still work:

How to enter IdeaCentre Q700 BIOS setting screen

To start the Setup Utility program, do the following:
1. If your computer is already on when you start this procedure, shut
down the operating system and turn off the computer.
2. Press and hold the F1 key then turn on the computer. When you hear
multiple beeps, release the F1 key.
To start the Setup Utility program, do the following:1. If your computer is already on when you start this procedure, shut down the operating system and turn off the computer.2. Press and hold the F1 key then turn on the computer. When you hear multiple beeps, release the F1 key.

go into bios and load to defualt setting
or
just remove the cmos battery turn it on then turn it off and put battery back in turn computer on then it should reset

Here is a link to the latest BIOS update for your computer available on the official Lenovo site:
BIOS Update for Microsoft Windows 7 32 bit 64 bit IdeaCentre B500 B505

That begs the question? Why update the BIOS? If you have a problem that the BIOS update will solve, Go to the Lenovo site and follow the instructions VERY CAREFULLY. If no, Do Not alter the BIOS. Altering the BIOS or Registry Editor is like taking a stroll in a minefield, rarely does any good come from it. And reversing a BIOS Flash/update can be a bit tricky, not to mention a real pain. Good Luck

Check your batteries

Look on ebay and search for Lenovo IdeaCentre K210. Your previous post said Lenovo IdeaCentre 5200. Which one is it???

I found plenty for the 5200. I only see one for the k210.

Welcome to Fixya.

Try booting in BIOS mode by tapping on 'DEL' or 'F1' key as soon as you turn the computer on (while seeing the LENOVO screen). When in BIOS, you can eject the disc.

Thanks for using Fixya.

Yes your pc should work properly as long as you have a proper power converter from UK voltages to US voltages, it's the same as a laptop