Well, I'm an avast reseller... it doesn't say so yet, only because I haven't made enough posts here to be able to edit all of my information. I have clients running avast! on Win7, Vista and XP with no problems. I, myself, run avast! on my two laptops, one of which runs Vista and the other running 7. This particular issue I'm having is with two client PC's. He's running 2000 on those two computers. He has no problem running the software on his XP machines, or on the other computer he has that runs 2000.
If Panda works better for you, then by all means, use it. I've never used it, but I understand it's a pretty solid piece of software.
I do, however, plan on finding a solution for my customer.