UPHClean Replacement

when/if you come accross problems with user profiles. you should enable userenv debug logging and take a look at what the output log produces - http://support.microsoft.com/kb/221833

and here's some additional information on how to understand the log format - http://technet2.microsoft.com/windowsserver/en/library/ccd7b430-99a5-40fd-b68a-6c1979e565a21033.mspx?mfr=true

also be aware that most of these problems reside with a process holding a handle on the ntuser.dat file. which is the user portion of the windows registry

you can try using a program like handle.exe to view what process is locking ntuser.dat, but chances are it might just show up as SYSTEM

best bet is to use procmon.exe and capture the registry events of a user logging in and out

fyi, none of this is "easy" but then again it's not supposed to be. programmers are supposed to make their programs unlock file handles when a user initiates a logon or logoff. so essentially you have to debug what a specific program is trying to do but can't do, or isn't doing.
also check out this page - http://support.microsoft.com/kb/314843/#XSLTH3129121125120121120120

it will describe how to edit the profilelist reg key, which is where the sid value for the local user accounts profile is to be mapped to. sometimes this can mess up depending on how you manage your user profiles and user accounts. if the user accounts sid is point to the wrong profile it can cause problems such as windows loading a temp profile cus it doesnt know where the local users profile is stored.

hope all this helps, and also be aware that most of the info above is for 2k/xp/2003. vista has a different method for dealing with such problems. i think is has uphclean built in, for one thing.