Your first problem is that you can't have more than 10 PCs connected to a Windows XP Pro shared folder. Thank you Microsoft. :) The best solution I see for you is to buy a simple NAS (Network Attached Storage). They are relatively cheap, and the few I've seen from Linksys allow you to set up usernames and passwords on the folder shares. They also allow you to select which users have access to which folders. You'll then have to remap all of the desktops to the proper shares. Usually, if you have set up the NAS device properly, you can use the same username/password combination that users already use to login to the local machine - so the network mapping is fairly transparent to them and they don't need to maintain another username and password. (this falls apart if you are enforcing a password change policy).
I've only used a NAS device from Linksys in the past and honestly was not impressed with its network performance. Hopefully someone else can chime in with a review of a decent NAS that will work in a Windows Workgroup type network like yours.