When booting, tap F8 a few times before the XP welcome screen comes
up.
You will get a menu of options, Select:-
Last known good configuration .... etc.
That should restore your system to a point where it worked OK.
If that does not work, then, after tapping F8, select "Safe Mode" and
see if you can boot that way. If so, start up "System Restore" by
clicking on "Help and Support" in start menu then clicking on the
"System Restore" line.
When system restore starts, make sure "Restore my computer to an
earlier time" is selected, click next, select a date before the
problem started then click "Next"