|Hitting the power button wasn't a good idea. You probably have disk corruption. Boot up using your XP install CD using the instructions |
here and type CHKDSK /F and hit Enter. The disk checker may well take a long time, so DON'T be tempted to power off or hit reset. It may be your only option short of a format & reinstall of XP. If it completes successfully, reboot, and hopefully all will be well. I've had to do this on several occasions for both friends and clients. You may get a problem with a program or two and have to reinstall them, or find that a file is corrupted. It's also possible you'll have to do a repair install of XP, but one step at a time is a good idea.
Any problems post back. Success - post back also, nice to know if it works OK.