Every once in a while, WMP craps out on my installation of Windows Vista x64. It happens when the 64-bit version of WMP tried to wrest control on my system's media playing abilities from the default 32-bit version. When that happens, not only does WMP stop working (and with a very cryptic and totally unhelpful error message, at that), but Napster stops working as well, citing error message 1002 (also extremely helpful).
The solution? According to this site, shut down any open media players, open a command prompt as an administrator, and type:
You won't need to restart your computer, just make sure you open the 32-bit WMP after you run that command.