I'm not a windows user per se, so perhaps someone else can best advise. However, some more information would be helpful for everyone.

Which codecs (or codec package) did you previously have installed (and version of codec if you know)?
Which codecs (or codec package) did you install recently that have caused this problem?

One thing I am aware of is that two different types of codec packages installed can clash with each other. So if updating or installing a different brand of codec package, it's best to uninstall the previous package if installed via package.

Thanks. I think it was the K-Lite codec pack that caused the problem. I uninstalled it, then installed Vista Codec Pkg 5.0.9. Everything seems fine now. I didn't realize that you uninstalled codecs using the same control panel that you use to remove programs. (duh!)