XP SP2: something's rotten

Some weird stuff is happening in my first XP setup. Problem 1: Sometimes it can take up to a whole minute to shut down, since there's a large time delay between each system tray item being removed (that's the only visual clue I'm getting). Problem 2: Sometimes, for no reason, visiting a website will make Firefox crash. It'll continue to crash every time it's re-run (or when it's re-run and as soon as it attempts to load a site) until i restart Windows. Problem 3: If Windows Explorer tries to read info out of an MPEG4-encoded avi, it crashes. (Codec issue? I can play these files fine thanks to ffdshow.) Couldn't open my Video folder for a whole hour. I've done a full antivirus and adware scan, and I'm completely clean.
EDIT: I went to post this and IE crashed. I'm posting this on a laptop

 

I can solve the long startups/shutdowns. All i added to my computer was an extra gig of ram. originally had 512mb now 1.5gb. I start up and shutdown process. Unfortunately i cant help you with the codec errors.

 

Ice - the problem with the shutdown issue is that it doesn't always happen. Most of the time it just flies through but sometimes it just lags along for no apparent reason, hard drive doing hardly anything. As for the codec problem, I tried uninstalling the MSMPEG4 codec (to be replaced by the ffdshow version) but no luck.