I was lucky enough to come into possession of a laptop that had previously been loaned out to a student and was no longer in use. Unfortunately, I couldn't find XP drivers for Sound or Display Adapter anywhere (not even on the manufacturers support site) so I had to install Windows 7. What a pleasant surprise. It starts up in hardly any time at all and closes down in about 3 seconds, unlike some Vista machines which seem to take forever. I have also noticed that it runs far fewer background processes - 38 on mine, instead of the close to 100 on my sister's Vista desktop.
I will stick with XP where possible, but I wouldn't have any worries about installing Windows 7 if necessary.