Hi, iv had this problem a few times, its probably because your boot.ini file is messed up.

Do you have the WinXp disk?

If so, try this-

boot to the XP cd and when you see the Welcome to setup press the letter R
You will get a dos prompt
Then type "chkdsk /p" without the quotes and hit enter
When that is done type "fixboot" and hit enter
"Y" and enter at the prompt
Then type "exit" and hit enter
The system will now reboot into Windows

This error can sometimes occur when a new hard drive has been fitted to an older computer, have you installed a new drive?

Hope this helps,

Lee