Free Windows Network User Accounting is the tool for the job. Though it is a bit of a challenge to set up, it's worth the effort. This web interfaced system is useful in finding out who is currently logged in and which computer they are using. The log is never cleared unless you clear it, so the history of who used which computer when can provide a lot of insight.
If you only have access to the local computer then checking the Security Event Log and checking the last modified times of the folders in c:\documents and settings\ can provide important clues.
If you are using NetBIOS (which is becoming less and less common) and you know the user's computername or IP address then you can use your trusty command prompt.
nbtstat -A [target ip address]
nbtstat -a [netbios name]