On a Windows Vista-based computer, you configure the following Group Policy setting:
   Delete user profiles older than a specified number of days on system restart
Therefore, you expect the user profiles that have not been used within the number of days that you specify to be automatically deleted when Windows Vista restarts. However, you notice that some user profiles that have been used within that period are unexpectedly deleted some days later.

This problem occurs because of an error in the User Profile Service. The Group Policy setting uses the time at which the profile was created instead of the time at which the profile was last used as the basis for determining when to delete the profile. Therefore, even if the user profile is still being used, it is deleted after the specified number of days have elapsed since the time at which you created the profile.

http://support.microsoft.com/kb/945122