Has it been working OK for the last few weeks?
If so, try and think what changes you have made (some programs update stuff without you knowing) between the time it last worked and the first time it didn't. (OK, basic stuff but it's easy to forget something simple).
I get problems (usually) after downloading something free or a trial version. Uninstalling still leaves stuff around.
I take it you have worked through the very long page that Dolbyman suggested?
The fact that you can still view stuff in Firefox suggests that it's a conflict somewhere - is there anything that you have updated (like the Flash player) in Firefox but not in IE? (You have to do that for both).
As a final suggestion, you could uninstall any codec packs you have downloaded, restart, see if that helps, redownload them (or just reinstall them if you still have the setup files) and see if there is a difference.
You could also try switching off the codecs one by one (see the previous help page).
It's probably going to be a long job but, as I don't have Vista, I can't be of specific help.
If you use Nero, you could uninstall (make a note of your numbers) and - most important - use their clean tools - all of them - to clear everything out of the system and see if that helps. If you don't use the clean tools, Nero leaves stuff everywhere.
Let us know what happens.