If you just installed Windows 7, added a large collection of files to your PC, then added them to the Library or as a folder in WMP (all in a short period of time), you need to allow the system to catch up - the files need to be indexed, data retrieved, etc.
This will take longer on older systems and even much longer on older systems with large libraries - or network shares with large libraries.
Is this the case?
An Index and Search troubleshooter is also available, type 'trouble' in the start menu to find it.
If waiting does not pan out, you can rebuild the databases for WMP and/or rebuild your indexes.
Let me know and I can provide data on how to do that, or search the forums for it.