Hrm. Provided you're downloading files from different places and all, it could be a problem stemming from a software/registry change. One option, therefore, is to use System Restore (if you're using WinXP, that is; I'm not familiar with Mac troubleshooting) and put it back at yesterday or, to be safe, a few days ago. If that's too drastic of a step for you, try uninstalling/reinstalling WMP before the Restore.
To help diagnose, though, have you installed any new software in the last few days?