Bluescreen While Playing TF2? Help!

[English Mobster]

This is a fairly recent problem; it began happening within the past 3 or so days.
Essentially, when I begin playing TF2, I can play for various amounts of time, each shorter than the last. For example, the day before yesterday, I was able to play for about 4 hours. Yesterday, it went down to 2. Today, I can only play for an hour.
After however long it takes me, I just get a pure blue screen. Not a stop error, not a BSOD. Just a blue screen, no text. Everything locks up, my music stops playing, and I'm forced to restart my computer.
Everything restarts fine, and I can leave my computer on for as long as I want with no issue; this only happens when I'm playing TF2 (I have yet to try it with other games).
I'm assuming this is a problem with my aging video card, although I'm not entirely sure.
As I said, this only began recently, I've played TF2 for hours with no ill effects before. It doesn't seem to be anything in particular that causes it; the other day I was going into a teleporter, yesterday I was behind a Sentry, and today I was crafting.
I don't understand why this has happened all of a sudden. The only major thing I've done to my computer in the past few days was reinstall Halo, and I don't think Halo has anything to do with it.
Dxdiag (yes, I have the crappiest computer on the planet):
OS: Windows XP
Processor: AMD Sempron 3000+ ~2.0GHz
RAM: 1024MB
DirectX: 9.0
Video Card: ATI Radeon 9600 series

This may be attributed to age. My sound card began flunking out on me a few months ago (although that may have been just bad speakers, as it fixed itself when I began using headphones instead of speakers), my USB ports are beginning to fail one by one, my computer's fan sounds like an old dog trying to get up the stairs, and I taped a giant industrial fan to the side of my tower to keep it cool.
I bought the computer in 2004, but the video card is an upgrade made about 2006-2007, just before I began really creating things for Halo (the old video card couldn't show water on Yoyorast Island, prompting the change ).
I've been able to play TF2 on this computer fine for the past 2 years, I bought the Orange Box in February of 2008 (according to my Soldier of Fortune badge), and it hasn't given me any hassle since.
This makes me a sad panda.

[Syuusuke]

So how hot does it get when you play TF2? (Use GPU-Z or HWMonitor or by feeling it up gently)?

[CrAsHOvErRide]

Typical symptoms of failing capacitors on the mobo. It's normal...your mobo is getting old. Though it's probably in the bus area of the gfx card.

[InnerGoat]

When was the last time you cleaned the heatsinks

[English Mobster]

So how hot does it get when you play TF2? (Use GPU-Z or HWMonitor or by feeling it up gently)?

Its getting pretty hot right now, and the only things I have open are Firefox and Steam.

Typical symptoms of failing capacitors on the mobo. It's normal...your mobo is getting old. Though it's probably in the bus area of the gfx card.

Right now, I'm getting this really strange "wiggly line" effect on my screen. My entire monitor looks like a JPEG with horrible compression. It's a pain in the ass to type this right now, as I'm relying heavily on Firefox's spell check.

When was the last time you cleaned the heatsinks

I don't believe I have in a while. I'll power off and clean them to see what happens.

E: I just did a test real quick (still haven't cleaned the heatsinks) and booted up into Linux (Ubuntu 10.04). For some reason, Linux isn't showing any of the same issues as I get when I'm in Windows. Could it be something with my Windows drivers? Or is it because Linux is a "lighter" operating system?

[CrAsHOvErRide]

Bluescreens are caused by two things: failing ring 0 devices aka drivers or failing hardware. You are probably right about Linux being lighter on the system and thus not crashing.

[English Mobster]

Well, I cleaned out my heatsinks this morning. A friend of mine suggested that I re-seat the video card, so I did that as well.
A piece of paper or cardboard or something seemed to be wedged where the video card met the motherboard. I'm not sure how it got in there, but somehow it did and was causing my video card's internal fan to not work. The card would get hot (due to the fan not spinning), and I would crash to a blue screen.
I removed the paper, the fan began running again, and I just got done from a 7-hour TF2 run to test it out.

Seems to work fine, so I'll close this topic unless it pops up again. Thanks for your help, guys!