Few possible problem by their importance,
* System Temp
* Hardware connectivity issue (due to dust or lose connect)
* PSU failing provide enough juice
* RAM Issue
* WinXP critical files are missing / Malware infection / BIOS issue
Now how do you troubleshoot them ??
System Temp
Make sure your processor fan is spinning, get in to bios and keep the PC on for certain time, then view the temp of system (CPU / NB / MOBO)...
Are they under limit ?
Hardware connectivity issue (due to dust or lose connect)
If your temps are in control, then detach all system peripherals, like RAM / GPU and clean the slots

then reattach them to see if that helps or not
PSU failing provide enough juice
even after cleaning the dust if the issue is still on, then try and borrow a compatible PSU from a friend of yours and try with that, see if you are able do normal operation or not !!!
RAM Issue
once again, borrow some ram from a friend of urs to see if the issue is still on or not !!!
WinXP critical files are missing / Malware infection / BIOS issue
Update the BIOS and then reset it to default

use BIOS option of jumper for ressting now install the trial version of a good AVS, such as ESET NOD 32 or KIS 2009, update and run a full system scan (scan all HDD drive and partitions) to see if any infection of not