Having multiple iTunes libraries on multiple computers can get pretty confusing. Not only have you purchased tracks on one PC or another, but often you've purchased them on a mobile device and synced or backed up that device to a single PC. Wouldn't it be nice to have the same iTunes library, with the same tracks, on all your PCs? MediaRover is a completely free solution and it works well.
Before you can get going with MediaRover, though, you need to set up file sharing. Start it on, say, your Public folder or a network drive, in Control Panel -> Network and Internet -> Network and Sharing Center. Password protect it if you feel that's necessary. Then, after you run the MediaRover wizard (and register for a free account; you'll want to do this as it's what allows you to share files between computers), it'll ask you for your Storage Server info. If you've set up a shared folder somewhere, click Locate Servers and it'll find the address of your network automatically. Then click Locate Shares, and it'll find all the folders you've set up sharing on for you to choose from. Put in the same password you just set up, then it's as simple as starting a sync.
Initially, this sync copies all of your music files to the storage server. You have the option of also copying specific playlists, or all of them. (MediaRover sees only static playlists, not Smart or Genius playlists.) Then, these files on the storage server can be synced to any other PC.