I open firefox and instead of my homepage I see a poor attempt to make me think I'm looking at "My Computer" and i'm supposedly scanning for viruses. I get a couple dialogue boxes telling me that I've chosen to run applications that I haven't chosen, and I can't get rid of it. I have to reboot the computer to get firefox to run normally. Anybody heard of this?

sounds like the antivirus 2007 virus.

malwarebytes usually cleans it right up in safe mode.

if not, format.

That happened before, but only from random deviantart pages.
Then I formatted and haven't seen it since.

Last night my media/server pc got infected with some malware after I attempted to watch a bogus movie. It completely made the computer unusable. I couldn't even open up taskmgr because it was "infected".

But it was pathetically easy to remove. Restart safe mode, deleted 324235325.exe from my docs app folder.

I had something similar to that that infected Windows security center. Besides scanning with a couple different anti-spyware programs, I also used a registry cleaner and a force deleter to get rid of any fishy files. If it was the same one as me, it'll just restart any processes you kill and reinstall itself if you don't clean all of it. It'll hide in C:\Windows and in certain folders in Documents and Settings. You'll also want to control whatever services it starts when you boot up. Just be vigilant and don't stop until you're certain you got it all.

Hmm. I just ran malwarebytes full scan, latest update, and it found nothing. What else should I try?

try combofix

If you know what you're looking for, run HijackThis and look for anything out of place.

Superantispyware.
http://www.superantispyware.com/

Profit.

If you're running windows why not try their security essentials stuff? I put it on my wife's computer and it seems to be fine.