With it being an old PC, could anyone tell me a resource to see which CPUs and what specification of DIMM will work - I tried to add a 128Mb PC133 DIMM a while back but it wouldn't work?
Try CPU-Z and see what it says about your MB & Ram.

When you tried the PC133 ram are you sure it was backward compatible with PC100 ? some PC133 isn't, but I don't know what your ram is....

Like others have said Win XP will work fine if you bump the ram up to at least 256mb. I have an old E-Machine with 128mb ram running XP in Windows Classic mode, it runs fine for general net type stuff...anything other than that it's running off the HD (page file=very slow) and its quite annoying