This is what I would do, and is by no means an expert opinion!
Backup the system using file wizard.
Make a restore point in XP
Run windows update, update the drivers for all devices, graphics, network etc.
Still problems flash your Motherboard (MoBo) to the latest Firmware, this can be very scary as it sounds like you may have MoBo problems anyway and it may fail rendering the PC useless.
If all the above fails reinstall windows, still got problems replace the MoBo, although it may be easier to replace the MoBO first rather than reinstall windows. Have fun