Some of my retrocomputer use requires partnership with a more modern winboxen to facilitate internet access for internet BBS use, or software access for storage of multiple disk images. These "partner" machines are commonly castoff 400+ mhz boxes which are retro by their own right, heading for curbside service. I've stumbled across an XP install called "MicroXP". Unfortunately, due to it's questionable legal status, it has to be obtained via bit torrent, but consists of a minimal version of XP that installs very quickly and initally uses 36mb of ram at boot. While it's ability to be updated has been disabled, it is current with both Service Pack 2 as well as security updates to mid-2007. It is light weight and perfect for rescue and repurposing old machines heading for the landfill. Google "MicroXP v0.6b" for more details.
On a related exporation, I've been very impressed with Microsoft's Virtual PC software, but it seems that an Open Source project has stepped forward to do it one better with a product called VirtualBox. They both have the ability to load various operating systems within a VirtualPC window on your desktop, but comparing VirtualBox to Virtual PC, VirtualBox wins with both faster install (almost twice as fast) and does not max my Pentium 4 processor when the guest operating system is idle. My next test will be moving my Virtualbox drive image into Linux and seeing how well it performs there. Interestingly enough, basic XP usage was faster inside Virtualbox that it was in the host environment. That is until I started playing Youtube videos. :)