Going by what I’ve been reading, Microsoft removed the ability to restore backed up DRM licenses in Windows Media Player 11 and later (Windows 7 has v12). As a result, if the software you used to play the music does not have its own ability to restore the license for music brought in from a previous PC / installation, the music must be obtained again.
As Napster does not seem to have its own support to import the music you downloaded on your previous Windows installation, unfortunately it looks like there is little you can do other than to redownload the collection. Most DRM stripping tools get the decryption key from the DRM license, while others rely on re-recording the music as some player plays it. As the license would have been lost with a fresh Windows installation and you’ve no way of playing the music, unfortunately there is no way for the DRM removal tools to re-record or decode the music. It’s a bit like having a CD collection with a padlock going through the spindle of each disc. If you can’t obtain the keys (i.e. recover the license), there’s no way of playing them.
Once you re-download the tracks, you should be able to use Tunebite (or another DRM removal tool) to strip the DRM and this is something I would recommend doing to hopefully prevent a repeat of this issue.
This is also why DRM has such a bad reputation and often nicknamed things like “Digital Restrictions Management” or maybe it should now be called “Disallows Recovering Music” or “Digitally Ruined Music”.