Change the boot order in your BIOS to boot from cd / dvd.
Then follow these instructions:
You can boot to the XP cd and when you see the "Welcome to setup" message, press the letter R.
You will then get a DOS prompt.
Then type: chkdsk /p [enter]
When that is done type: fixboot [enter]
Type: Y at the next prompt [enter]
Then type: exit [enter]
The system will now reboot into Windows.
If for some reason that didn't work, you can boot to the recovery console (example above).
Type: "chkdsk /r" [enter]
When done type: exit [enter].
This takes a bit longer, but the system should boot back into Windows.