Use GPO to force a logoff script and write a vbscript that does something like this:
Set FSO = CreateObject("Scripting.FileSystemObject")
Set Net = CreateObject("WScript.Network")
Set LogFile = FSO.OpenTextFile("\\server\share\" & Net.ComputerName & ".txt",8,True)
LogFile.WriteLine Now & " - " & Net.UserName
FYI - I freehanded this and didn't doublecheck anything, so you may need to do a little Google work to get syntaxes right. Or reply to this an I'll test it for you.
Regards,
Scott
http://scottisageek.spaces.live.com