I see in the specs you have 4 PCI-E x1 slots. And 2 PCI slots. See if best buy has a PCI-E x1 network card, if they don't just get a PCI and put into one of your PCI slots. I'm definately sure they'll have vista drivers with the card.
A LONG time ago I had a PCI network card failing. It would only hold the internet on campus internet for 5 min before it died. New card fixed it.