The fix will depend on exactly what is wrong. The messages mean that your computer can't read the hard disc and find a boot OS. The drive may have failed, the cable might be loose, or the MBR (masterboot record) is bad.
First make sure that you do not have a CD/DVD in your optical drive, a floppy in your 3.5" drive or an external hard drive attached to one of the USB ports. (The computer may be trying to boot to one of them.)
Next press F2 when you turn on the computer (press repeatedly). This will let you enter the BIOS to set the disc boot order. (Check that the hard disk is listed first. This also indicates that the computer sees a hard drive in the machine.) If you get the error message again, try the entire procedure again from turning on the computer.
Next try using the appropriate Windows install/update disc to repair your system.(The Dell 4600 came with XP and you need the correct SP on the disc. If you kept up on the updates, it should be SP3.) Put the CD in the optical drive then restart the computer. Press F12 repeatedly to get the boot selection menu. (The same repeat routine may be needed if you miss the timing.) Use this disk to enter the Recovery Console and attempt to repair the Master Boot record.
Use the directions here: http://support.dell.com/support/edocs/systems/dim4600/en/4600i/sm/index.htm
to check if the cable is connected properly. You will need to open the case.
This Dell came with an IDE hard drive. These often have a jumper that indicates the boot order. The jumper must be set to master or cable select and be attached at the master position on the IDE cable. This is the connection at the very end of the cable. The IDE cable can come loose from the motherboard, as well. You can also try a different cable.
The final option is to replace the hard disc. Unplug the old hard drive from the IDE cable first. You'll need the Windows install disc and then to reinstall all of your drivers and programs. Move the bad disc to the slave position on the IDE cable. See if you can salvage any of your files from the hard drive. (Sometimes, putting the drive in a ziploc bag and placing it in the freezer for several days will let you read the disk when you put it back in the computer.)
I hope this helps.