Ted, I've had 9 OS's at one time, it's only really limited by the space you have.
Actually, at one time back in 03 when I was still working in the anti-malware field, I had 2 W95's, 2 W98's, 2 W2K, 3 XP, and another 6 or thereabouts in VM...heheh. I had one of each OS used as strictly isolated clean installs that I would install an unknown malware (or suspected malware) and they were all monitored for every change made. Those would be reinstalled immediately after it was no longer needed to monitor that variant. The others would have certain config changes made and then the malware would be installed and tested on that for other certain behaviors like port sniffing them to see what/where they wanted to contact.
Then the VM's for any real nasty that needed to be completely sandboxed and abused, and the rest for my use.
I tell ya, I got tired of installing OS's after years of that...
@ Digger - I'm actually finding the opposite, I find 7 to be really fast to a working desktop even with gadgets (4), AppLauncher, Weather (default one), Calendar and one dual CPU/RAM monitor. Vista was a horrible Dawg with any gadgets running on startup...up to 2 minutes after the Welcome screen started, almost enough time to make coffee

I think they got it fixed with 7 now. At least for me anyway