It’s called the User Profile Hive Cleanup Service. It clears up lots of memory management issues. Helps on everything 2000, XP, and 2003, most definitely (sometimes especially) including terminal servers. Speeds up any workstation or server. Apparently it helps keep the registry small in real-time. Not for Vista, there are notes on that subject within.
http://www.microsoft.com/downloads/details.aspx?FamilyId=1B286E6D-8912-4E18-B570-42470E2F3582&displaylang=en
There are very rare instances when the registry on a PC or server is corrupted in ways which the UPHCS can’t clean, and in some of these cases, we must boot to safe mode and disable the UPHCS. I have seen only three such cases out of many hundreds of installs to date.