I just discovered a neat little tool from Microsoft - designed to prevent those OHHH - SOOO-SLOWWWWWW shutdowns with multiple user profiles.Its the sparklingly-named User Profile Hive Clean Up Service [fairly trips off the tongue, doesn't it ?] - but it seems to work a treat.
No, it does not: its only for the 32-bit versions of XP and Server 2003. At a quick look I could not find an equivalent one for Vista, though maybe its early days yet. It was an awful long time after the introduction of Windows 98SE that M'soft addressed a collossal shut-down issue (actually probably being unfair to M'soft - I guess they just could not find the answer).