Virtual Server 2005
Virtual Server 2005 is an unusual specimen based off of Virtual PC, that is designed to be a server variant of Virtual PC, except for the fact it's more of a pain in the ass to use and requires you by default to install an vulnerability on your computer (IIS 5.x) in order to be able to use it.

Due to the by-default IIS 5.x requirement, this only works on Server 2003 and XP, it will not run under any other operating system due to compatibility issues and the short lifespan of the program itself. (tl;dr no one used this piece of shit because it sucked too much)
You can actually avoid the requirement of having IIS installed if you disable installing the web interface in a custom setup. You would then need to use a client such as VMRCplus to configure your server. This may come at a loss of possible features, but I have not seen a parity mismatch between the two.
VMs are controlled by using the VMRC protocol (modified VNC with NTLM/Kerberos slapped onto it), however unlike regular VNC it's a lot faster in terms of screen updating in some cases even on really bad connections.
Virtual PC and Virtual Server VMs are both compatible within each other (as long as IDE is used) as the base virtualization component is identical to it's Virtual PC counterpart.
Differences between Virtual PC and Virtual Server
- Virtual Server doesn't support sound.
- An Adaptec SCSI adapter is emulated alongside IDE.
- Instead of an native Win32 interface, the main interface is powered by ASP.NET, however there's a native win32 client for only controlling the VMs themselves.
- Hard disk image sizes are limited to 128GB.
Bugs
- On Windows XP hosts, you have to disable Simple File Sharing because due to a bug it tries to authenticate as the guest account for whatever reason (because it's ASP.NET)
- The web interface only works on IE6, or under compatibility mode under IE8.
- The SP1 update includes a broken DCOM component, which has broken authentication. (workaround is included in screenshot)
- It only reads floppy images when the file extension is .vfd, if you try to mount a file with any other file extension, it will error out that the file is missing despite it's present on the directory.
- The Mystify screensaver doesn't render correctly on the VM display.

How to use on CollabVM
A depraved individual has managed to implement working Virtual Server 2005 support into CollabVM 1.2 (TS). The source code can be found here.
(This exists only as a joke and you do not ever want to do this unironically.)
The web interface is currently required to be installed for this implementation.