This is one of the weirder problems I've ever had

I have an odd issue, I'm not even 100% sure it's a Vista problem, but I'll start here.

Essentially my computer, usually at random, just freezes. Mouse won't move, no keyboard response, any audio playing bugs out and just makes a loud noise until I restart the computer by holding the power button.

It has only happened when I am browsing the web, never during a game or any offline applications. It has happened while using Firefox and while I was using IE (just as a test to see if it would happen too).

The only way I have been able to replicate the error is by running internet speed tests. Specifically from speedtest.net and pcpitstop.com. It happens exactly once the download portion is right at the end or over, and right before the upload part begins. I can upload files with ftp programs no problem.

I built this computer just about 7 months ago and have not had a single issue with it until this started yesterday. Here is a list of things I've done so far:

1. Check and rechecked with anti-virus in and out of safemode.
2. Check and rechecked with 3 different anti-spyware programs in and out of safemode.
3. Defragged.
4. Check HD for errors.
5. Used CCleaner to clear up temp files and registry issues.
6. Checked for driver updates to my wireless NIC.
7. Cleaned out inside of the case and run coretemp to make sure the CPU isn't overheating. The hottest of the 4 cores topped out at 44 degrees even after an hour of gaming.
8. Downloaded an installed all windows updates.

Has anyone ever heard of anything like this? Thanks for any help. I have Vista Home Premium by the way.

Asus PQ5-Pro
Intel Core 2 Quad Q9550 2.8GHz
VisionTek Radeon HD 4870 512MB
Mushkin (2 x 2GB) DDR2 800
WD Caviar 640GB HD
Vista Home Premium SP1 64-bit

Linksys Wireless-G PCI Adapter (Driver version 2.1.3.0)

I was able to run a couple speed tests this morning without freezing, but after that it froze one time when opening youtube.com.

Everything seems to be set to the correct values. I haven't changed anything though so I don't see how it could have just started messing up. The default is set to 1.8, but that is what Mushkin says it should be.

The fact that it only happens when I'm using a web browser makes me think it's software related, but I can't find anything.