Try repairing the windows XP installation as follows:
Click start
select run
type in cmd and click ok
enter the following command in the new window that opened followed by enter
SFC /scannow
insert the windows CD when requested.
That will repair any corrupted files or those that have been replaced by third party software with older versions.
If this doesn't correct the problem try restoring the system back to a date before the problem started:
click start
select all programs
select accessories
select system tools
open system restore
Follow the prompts on screen.