Upgrading Old PII for short term use.

The easiest way to tell is measure the existing one. This is an old case you're using is it ATX and not AT?

The motherboard (Intel RC440BX) is MicroATX, does that help? Also, if this means I need a Micro ATX power suppy, where ca I get one, the only one I can find on Overclockers.co.uk is a 285W Enermax, which is great but probably not prowerful enough.

Get the ruler out and measure what you have already. If it is micro-ATX you possible won't find much bigger. Enermax are pretty good I use them in two of my machines. Maplin often have some, although there doesn't seem much on their website at the moment.

HardForum will tell you what are the good makes and more especially the ones to avoid.

I measured it, its' an Astec ATX 200W. The dimensions are...
Length- 16cm
Width- 7.5cm
Height- 12cm

I have a feeling it may have been on its' side though, but still, there it is.

You only need an ATX PSU then.