<<Are you sure it would be a hardware problem?>>
Did I say that? Of course, I'm not sure...but every journey has to start somewhere
Generally...I find that XP will generate a cry for help when most things go wrong with the O/S. Note that I did not say "the system."
The one exception to this premise...would be corrupt system files...and, even then, error messages are generated when "important" system files have been found to be damaged/corrupted.
I am not saying that XP finds/reports on every instance of damaged system files...I don't know that at all. I just know that there is an incredible amount of documentation available on just about any recognized damaged system files.
Typically...if an application results in a crash/reboot...the system will record the error onscreen or in Event Viewer. Hence, the link.
Lacking any sort of help or cry for help from the system via the O/S...I have to resort to just guessing.
Hardware is the big area where XP cannot help itself or the user very much. It can detect certain problems caused by drivers, but only in a general way. It can detect certain problems in memory...but won't necessarily pinpoint the cause of the problem. It can detect certain problems with hard drives, but not with the precision of the diagnostic programs available at the websites of hard drive manufacturers.
Heat-related problems are detected only because it's a self-preservation scenario (at least, that's the way I look at it)...the system turns itself off.
File system (NTFS or FAT) problems are sometimes detected...often not. More guesswork, with the ultimate way of dealing with such being a reformat/clean install (since everything installed depends on the file system).
Since I believe that you have an XP install CD...you can try a repair install and hope that takes care of whatever is wrong. Shotgun approach and it won't be effective if it is a hardware or file system problem.
You can easily test the hard drive...you can easily test the RAM...so I would start with those two.
And...as always...it could be malware-related. I don't touch on that much because it's not an area I know much about, other than how to keep my two computer systems fairly free of anything other than adware/spyware which is removed quickly.
In any case, I hope you see why I stated what I stated...in the manner that I stated it
Systems are complex.