This is a friend's computer. Almost 3 years old. Dell Dimension E520. Windows XP SP3. All Window updates up-to-date. 1Gybyte memory. 228GByte disk, 201GB free. Microsoft Office 2003.

Used by two high-school girls who only surf the net, sync their iPods, and use Word and Powerpoint.

SYMPTOM 1: The system hangs sporadically and upredictably. Sometimes one a day, sometimes thee or four. Sometimes when the girls are using Word or Powerpoint, sometimes when they're Internet Explorer, sometimes when they're not even doing anything but have simply walked away from the computer and returned only to find the desktop "frozen".

SYMPTOM 2: The girls also reported that there were "lines all through the display".

Other than the hanging/freezing problem, the system seems to be running fine. That is to say, the performance is good, no problems accessing the internet, etc.

WHAT I'VE DONE:

First the obvious:

Ran adware, spyware and antivirus programs: AVG AntiVirus, MalwareBytes, Spybot Search & Destroy, SpywareBlaster, Adaware (all but MalwareBytes were already on the pc and were kept up-to-date regularly). I ran all scans for each user (i.e., logged in as each user, than ran the scans). The antivirus programs did find and clean a couple of Trojans. I keep rebooting and re-running the scans until they ran clean. I thought cleaning off the Trojans would do the trick, but alas, no.

Checked that they had plenty of free disk space - they do.

Given the problem with "lines on the screen", I updated the video driver (nVidia Driver GeForce 7300 LE 128) - but this had no affect.

All of the above was done remotely using LogMeIn, and let me say this: while I was doing all this work, the computer never once hung on me, EXCEPT for one time when I was downloading the new video driver. The above work represents about 12 hours total of on-the-computer time, broken up into pieces, with some stints as short as an hour and others up to 5 hours.

THE NOT SO OBVIOUS THINGS:

Finally, I had to go on-site. I ran the Dell-supplied diagnostics for troubleshooting "System Hangs" problems. This included intensive memory and disk diagnostics and took about five hours. The computer never hung while these were running, and none of the diagnostics reported any problems.

Ran the video diagnostics: no problems found.

While I was there, however, the Mom reported that just before the system freezes, it's "really loud" and the computer gets really hot. So I asked, "really loud as in a fan is working hard" and she said yes. That's when I discovered that all of the air vents were fully clogged with dust! AHA, I thought - that's the problem. The computer is overheating. I vacuumed the thing out thoroughly (removed side panel and got inside, too).

I noticed the "lines across the screen" and looked into that. They were using an LCD monitor with an analog connection. I switched it to digital (DVI), since they had the cable and the computer had connections for both. Lines were still there. I went into Control Panel > Display > Troubleshoot. Hardware acceleration was "Full". On a hunch, I moved the slider to the middle - that got rid of the lines.

I left after being on-site for eight hours without a hang, confident that I had finally found the problem!

The Mom reported the next day that the computer had hung again that morning. Aaarrrggghhhhh!

SO - I'm looking for suggestions about what to try next!

Possible ideas - driver updates:

Bios: Dell-BIOS (recommended) - dated 7/11/2007 (I didn't install this because I wasn't on-site, and then when I was on-site, I thought sure I had solved the problem; I'm always a bit leery of updating the BIOS unless absolutely necessary).

Dell Driver updates (this is what Dell reported as "recommended" or "urgent" - I have only done the Video driver so far):

Chipset: Intel - Driver (urgent) - dated 6/25/2008 (I didn't install this because I read about it and it didn't sound relevant to the problem)

nVidia Driver (GeForce 7300 LE 128) - recommended - dated 6/11/2007 (I have installed this one)

Sonic Solutions-Patch/Upgrade (CinePlayer 3) - urgent - 1/2/2007 (I didn't install this because they don't use that software)

SATA Drives:
Intel-Application (Matrix Storage Manager) - recommended - 6/12/2007
Intel-Driver (Matrix Storage Manager) - recommended - 1/24/2008

I'm not looking for an answer as simple as "yes, install all the driver updates". If you think I should install a driver update, please tell me why you think it will help. (Thanks)
I'm looking for some constructive help on what you think the problem could be - what have I overlooked?

If you've gotten to here - thanks for reading all of this!