Normally we don't cover classic ASP, but we are making an exception for a potentially mission critical bug. ASP has a serious bug in handling the Session_OnEnd event on machines running Windows Server 2003 with Service Pack 2.
According the Carlo of Microsoft, the Session On-End event will simply not fire. The current workaround is to remove SP2 from your server, though a hotfix is currently being tested. More information in the form of a KB article has promised.