hi, there is a tool in the NT server resource kit that can produce a report of usernames/last login date/ last login server etc. I use it myself to identify unused accounts
I cant remember the exact *.exe name though! have a look through the resource kit notes to find it.
Also, you may be tempted to just delete the accounts that havent logged on for a long period. My advice is to disable them first to ensure that no problems are caused etc (i.e. it may be a system account that a service uses that is not documented etc.