According to Windows XP Inside Out:
On the computer with the shared profiles folder, right-click the new profile folder, choose Properties, and click Security. If one of the names shows the security identifier of an unknown user, you must add the correct user account and give it Full Control permission. You can remove the unknown user, although it’s not necessary to do so. (The unknown account is actually the correct user name, but it’s the account from the source computer, not the account on the local computer. This is one of the hazards and annoyances of relying on separate security databases—as the workgroup model does—rather than using the centralized security database used by domains