I haven't asked why you want to avoid buying XP - is it just cost. It is only my personal opinion but I think XP was the the best quality OS to come out of MS so far. Maybe Vista will eventually become that but I think it has a way to go before it overcomes all it's bloatware.

One thing you may be interested is that, from using 2000, the internal directory structure and architecture of XP will mostly be familiar to you. With Vista it is very different. There seems to be a virtual directory structure overlaying the real one and I found this quite hard to get to grips with.

In terms of your PC running slow, I would say that you should try to have an OS of the same generation as the PC. This way the the power requirements are likely to be matched by the equipment that was being sold at the time.
If you want to go for XP then you should be able to get a very good deal on a genuine version if you can find anyone still stocking it.