one of the programs you are installing is buggering things up - more than likely be an older 3rd party program. The only thing I can suggest is loading all the safe programs first, such as MS Office - then one-by-one slowly introduce other programs and make sure the system is OK after each introduction. It could even be a driver/software package that was not intended for XP that works close to the system, and therefore requires the version of windows for which it was intended.
Also read the systems requirements of each program you install - look specifically for ones that state they will not run on windows NT, 2000 or XP.
If you are at a loss to which it may be, then list the names of your software and give a brief discription of the less known packages.
Peter