I suspect upgrading is the biggest cause for Vista's problems. From the people I've talked to, Vista does not do well with less than 2gig of ram, while XP cannot access more than 2gig. And, if the hardware was designed for XP, the updated drivers may not be as effective.
One thing is certain: Hardware and Software vendors will eventually stop developing on the XP platform. Sooner or later, the Vista will be a better alternative than XP. I'm hoping Linux or Apple will be better than Microsoft.
I have been using the full version of Grisoft AVG on XP. I've had only one attack and the software took care of it cleanly. On this machine, I'm trying just the free version and so far, so good.