XP Event Viewer is LOADED with errors

WinXP Home SP2 - This PC had the HD formatted and a clean XP install done on July 1st. The owner of this PC was complaining that it would lock up at random and/or shut off at random, so I run the usual tests (thermal, check PSU, malware, etc). It appeared to be a low-level driver issue that I couldn't pinpoint and the owner said "go ahead and format, I have everything backed up". So I formatted, installed XP and drivers, and all was well. I let the PC run continually over the July 4th weekend just to be sure it was stable. It was. It ran fine, no crashes, no lock ups. Now the PC is back in front of, and again the owner is complaining of random lock up and crashes/reboots. Yesterday afternoon (about 4pm) I powered it up and again let it run all night. No problems. This morning it was still running fine with the same web page open on the desktop. Upon checking the Event Viewer, I found the same error occurred DOZENS of times in the span of maybe 10 seconds on July 7; it started at 1:33:45am and errored out about 15 times per seond until 1:33:56am (screen shot attached). The error properties are:

The error appears to have 'solved' itself; by 1:34:18am, this error is no longer occurring, and does not make another appearance anywhere in the event viewer. Any ideas? Thoughts? Suggestions?
Thanks.

 

The only thing added since the format/clean install is HP printer drivers and software, and Office 2007. That's it. It's weird.... the guy says it reboots at random, but everytime I have it, it never acts up (well, it did the first time, back at the end of June and that's why it was wiped and reloaded, but since then, it's been fine every time I look at it).
:confused

 

Hmmm... yeah, good ideas.... I didn't think about it maybe being crammed in to a small cubby hole in a desk or something..... thanks!

 

Here's an update: I removed the 56k dial-up PCI modem since it was unused. I disabled the on-board ethernet and installed a 'generic' RealTek 8139 PCI ethernet card. This seems to have solved the problem. I think maybe the on-board NIC is somehow faulty, and since installing different drivers made no difference, I'd say it's a board level hardware problem. The owner of the PC doesn't really want to spend any more money on this aging platform, so a mobo replacement was out.