I have rebuilt XP pro on two computers now using a very old disc with a copy which hasn't even got SP1 on it. When I get it up and running, XP flies. Then it gets slower and slower as the huge number of mods and service packs are incorporated.
I suppose in an ideal workd one needs two computers: one with an unmodified XP pro to run today's fancy software but without internet access, and one running, say, Win '95 or Win '98 which modern viruses are not written to infect.
P.P.