Your library and the physical song files are 2 different things. The library is just a database of song information like the files location.
If you go to your iTunes folder, you'll see a file called
iTunes Music Library.xml. This is an xml copy of the library database for other applications to use. Open this file with IE and do a search for one of the songs where iTunes says unknown location. Look at the Location tag to see where it thinks the file should be. Go to that location in Windows Explorer and see if there are any songs there.
This is what a songs info looks like in the xml
PHP Code:
<dict>
<key>Track ID</key><integer>1870</integer>
<key>Name</key><string>Award Tour</string>
<key>Artist</key><string>A Tribe Called Quest</string>
<key>Album</key><string>Midnight Marauders</string>
<key>Genre</key><string>Hip Hop/Rap</string>
<key>Kind</key><string>MPEG audio file</string>
<key>Size</key><integer>3765569</integer>
<key>Total Time</key><integer>226690</integer>
<key>Track Number</key><integer>3</integer>
<key>Year</key><integer>1993</integer>
<key>Date Modified</key><date>2008-08-10T22:07:48Z</date>
<key>Date Added</key><date>2006-09-16T20:00:31Z</date>
<key>Bit Rate</key><integer>128</integer>
<key>Sample Rate</key><integer>44100</integer>
<key>Play Count</key><integer>10</integer>
<key>Play Date</key><integer>3322101767</integer>
<key>Play Date UTC</key><date>2009-04-09T12:02:47Z</date>
<key>Rating</key><integer>100</integer>
<key>Album Rating</key><integer>100</integer>
<key>Album Rating Computed</key><true/>
<key>Artwork Count</key><integer>1</integer>
<key>Sort Artist</key><string>Tribe Called Quest</string>
<key>Persistent ID</key><string>80B9A3690CAF8D8B</string>
<key>Track Type</key><string>File</string>
<key>Location</key><string>file://localhost/Volumes/iomega%20red%20hdd/iTunes/A%20Tribe%20Called%20Quest/Midnight%20Marauders/03%20Award%20Tour.mp3</string>
<key>File Folder Count</key><integer>4</integer>
<key>Library Folder Count</key><integer>1</integer>
</dict>