AVI problem in Windows Explorer, Media Player and Real Player

Hello,
I have Windows 2K Professional and IE6.
This is a new problem. Prior to a year ago the problem didn’t exist.
For the last year or so, AVI files have given me trouble. When I roll the mouse over any AVI in Windows Explorer and the preview tries to initialize, Windows Explorer locks up.
In Real Player or Windows Media Player, if I try to play an AVI it’s fine till I touch it. For example if I try to fast-forward in the clip, or switch to the next clip, or stop the clip (any of those things), the player locks up. If I pause and resume though, it’s fine.
Is there some codec that I’m missing? This is so annoying.
I used to be able to put all my video clips (all except Quicktime files) into the media library in my Windows Media Player and play them all back to back, with no problem. But now I can’t. As soon as an AVI plays, trouble starts.
Any help would be appreciated.
I have other players which will play them. But I’d like to be able to use my Real and WM players like I used to.
Thanks.