I have a bunch of not very new PCs, macs and an HP box here.
I have build my own system (IronBox Linux), that is already used by some companies for high security and non stop stations and high productivity servers.
About how often do I change my PC ? Well, my wrkstation is a PII at 350MHz, same as my test server. Of course, there is a mosix cluster on my network, so when I need more power, I just add a new node to the cluster or rearrange priorityes
Last time I bought a box... I think it was one week ago or less.
I have only one windows box and I use it to play old games. I have never worked seriously with Windows XP or Win2k. I guess that an operative system that sucks 128 MB to say only "helo" is not worth the effort even to install it.