Buy a new PC??
That's only slightly a joke, as WinXP is now end-of-life and if you have to do a reinstall, it's going to be bloody hard work to get hold of all the patches and whatnot. Win7 would most likely run achingly slowly on WinXP grade hardware.
Anyhow, if the thing won't boot even into safe mode or command prompt then you may have some joy by putting your WinXP CD into the drive, then booting from that and trying to repair your current install.
If the boot order needs changing so that the CD drive is checked before the hard druve, then you'll need to push the 'del' key as soon as you power on the PC and make the change in BIOS.
All I can suggest I'm afraid