Sounds weird, if you have the normal version (non-portable), both plug-ins and media are stores in a folder in your user profile - you can get there by typing %APPDATA%\RealWorld\RWPaint in Windows file manager.
Is there enough space on your harddrive? Have you installed something that could interfere with other programs?
You can try the portable version instead, it stores plug-ins elsewhere, maybe it will work.