It's a Hewlett Packard, but I've modified it quite a bit since I bought it - I think I still have the original manual somewher, and if not I can probably find it online.

I also suspect it is the power supply, but I bloody hope not - I don't need any unforseen expenses right now, but I do need my PC back ASAP.

It could be a dying hard drive - one of the 2 I have in there I took from an old piece of junk my brother was throwing out. If I was to remove the offending device might this help?