In Windows, you are either in a Workgroup or you are in a Domain. You have no other choices.
A "Workgroup" in Windows is only a convenient organization tool. Whether someone is in your Workgroup or in a different Workgroup means NOTHING. The only thing stopping others from accessing your computer are Sharing and Security restrictions.
If you change your XP Professional to "non-Simple-Filesharing" mode, you'll have complete access control on your PC. Taking advantage of User Accounts, Passwords, and Sharing/Security Permissions, you can choose who can access data on your PC.