in order to even guess at what the problem is, you will have to post a more detailed description of what is, or is not happening when you try to start your computer.
What does the screen look like, what picture is on the screen, or is it a blank, black screen?
Try this for starters... turn off the power to your computer, unpliug your computer and wait a minute, then plug it back in and press the power button. If nothing changes, read the link provided,
http://support.microsoft.com/default.aspx?scid=kb;en-us;819038&Product=winxp
To access your BIOS, power off the computer again, wait one minute, the press the power button to restart your computer, and, as it begins, press the Esc key, that should bring you to the BIOS page.
