A colleague at work installed IE7, on his machine it killed the cd writing software (Roxio) and broke his Yahoo toolbar stuff. He went back to a previous restore point in XP, and it took nearly 2 hours to revert the machine.
Personally I only use Firefox for browsing the web, much more stable than anything MS put out.
In fact I don't run anything MS (apart from my keyboard!), as I use Linux instead of windows.
