Why does your computer reboot when it tries to start Windows XP professional? Power on that computer and immediately start pressing an F8 key until an advanced boot options menu opens. Choose "Last known good configuration" from that menu.
Hopefully, Windows XP will boot normally. If it does not, then go back into that advanced boot options menu and choose "Safe mode with networking". Open up a command prompt and type in "chkdsk /f /r" without quotes.
Type Y at a volume is in use message. A chkdsk scan will be scheduled. Reboot that computer and a chkdsk scan should begin.
If you don't know how to open a command prompt window then click "Start" then type in "cmd" without quotes in a "Run" box. Hit enter and a black command prompt should open. You need to start a chkdsk scan, it shouldn't take more then a minute to kick it off and then reboot.
Also, make sure you blow out any dust from your power supply fan and case fan if your computer freezes. If you have a Windows XP installation disc, you can run a chkdsk scan with a recovery console that you can boot to off that disc.