Blast from the past: Windows 2000 Server
Finally getting around to installing an old copy of MS Small Business Server 2000 I got from attending an MS training course years ago. Using MS Virtual PC, which seems quite nice and runs fine on Vista. The funniest thing I've seen so far? I'm used to seeing multiple gigabytes of memory consumption in Vista's task manager, but Win2k server is humming along quite nicely using under 150 MB :-)
Update: after installing Exchange 2000, SQL Server 2000, Active Directory and all that it was up to more like 300MB. Oh well...