What is the oldest Windows operating system to run on a modern (2007-2009) computer? [closed]

I have Windows 98 in a VirtualPC on my home machine. I use occasionally it to run a Windows 3.1 era copy of MathCAD which just isn't happy running under XP, and to play a couple of ca. 1995 games that really want an early version of DirectX and can't handle XP either.

I also have a DOS 5 in a VirtualPC, left over from the steps required to get Win98 installed in the first place.

Edit: One advantage of a VM like VirtualPC for this is that you can easily present the ancient guest OS with an appropriately limited set of resources.

I've successfully run Windows 3.11 for Workgroups with DOS 6.22 on a spare 2Ghz Celeron machine I have. Had to install a seperate TCP/IP stack for network support but using IE5 I did manage to get the little thing on the internet. :-)

If you have any sort of virtualisation product installed, there is no limit!

I have messed around with Windows Disk Operating System (before it had a version number!), and many other systems that are 20+ years old.

If you are willing to take out some RAM, you can get Windows 3.1 working no problem on modern hardware - It will work with the extra RAM, but HIMEM.sys can take an age to complete on a high end modern system!

I know you can go earlier than that, but without any sort of hack and just the standard disks, I think that Windows 3.1 / Dos 6 is probably the easiest to install early OS that anyone can mess around with.