I found the solution to this in the PsTools forum:
"Apparently, updates to XP (most likely Security Updates delivered via Windows Update) now set a local security policy setting differently than in the past. Do Administrative Tools->Local Security Settings->Local Policies->Security Options and look for the line "Network Access:Sharing and security model for local accounts". As of at most 6 months ago, the default value for this "key" was "Classic - local users authenticate as themselves". But new XP installations and those with recent security updates will have a default value of "Guest only - local users authenticate as Guest". Change it back to "Classic - local[/indent] users"