I think the answer might be the Windows Media Player and Windows Media Network Sharing Services as zxypher suggested. The fix for that is to adjust were the Windows Media Player services search for media files.
It seems to be a bad problem for me as well upgrading from Vista to Windows 7. The default search location for media is set to C:\Users\ instead of down in the My Music folders.
Go into
Media Player. From the menu ->
File ->
Manage libraries -> select each library (music, video, pictures, recorded tv [if you have]) and check the paths that it is managing and make sure it is not too close to the root of the C: drive. For example, it should be something like c:\users\Jim\Music and c:\users\Public\Music
BUT NOT c:\users as they will cause the whole directory tree to be re-indexed over and over (when you surf the web or change any files in your profile).
If you have multiple users who login to the computer, you may need to repeat the process for each user if all users have added managed folders which are actively changing on a regular basis.
If this is your issue and you do it correctly, then the high CPU should drop in minutes (or seconds down to nothing).
This is the CORRECT way to adjust the WMNSS/WMPNetwk.exe process issues.
A little more information can be found here:
http://social.answers.microsoft.com/Forums/en-US/w7performance/thread/2fe4901b-2eaf-4a99-b487-e6a6213b10b3/
