Only 3 years after the release of SP2 no less...

Download and install the latest hardware drivers for your:
- Motherboard chipset
- Audio device
- Network card
- Video Card

Only you know what these devices are (along with those who originally sold
you the system - if it has remained unchanged since that time) and the
system itself.

Short:

- Boot into Safe Mode and remove SP2 (System Restore should allow you to do
this - or booting into Windows XP Safe Mode and Control Panel --> Add or
Remove Programs...)
- Reboot into normal mode and figure out what hardware you have (Belarc
Advisor can help you there - http://www.belarc.com/ )
- Visit each of the hardware manufacturers web sites (if you have a
name-brand PC - Dell, HP, IBM, etc - you may be able to get all the drivers
at one place) and find the latest Windows XP driver for each piece of
hardware and download/install them.  The Chipset should be the first one you
do - the order of the rest is unlikely to matter.
- Install SP2 again - after you have installed all the drivers and checked
your system for spyware/adware, CHKDSK'd and Defragmented and backed up your
important data (files, folders, documents, spreadsheets, emails, contacts,
favorites, etc...)

Come back and let the newsgroups know your progress or if you need more
assistance.