When it comes to MAC's you should talk to Shane about his, its got another problem now.
Talking to some friends that do desktop repair for a living, they have yet to deal with a virus issue on Vista.
I would suggest staying away from a laptop unless you plan to use it on the road, they are designed for a specific type of use not for the home user that plants it on a desk and leaves it there. Its a waste and your going to pay more for upgrades and parts.
Buy yourself a brand name box with a decent warranty, your not really a power user, you dont need a kick ass computer, you dont need Unix/Linux.
Desktop computers are cheap, the parts are cheap, I could build you a box cheap and you'd be on your way.
If you want a brand name, I will say go with HP and get an Intel based system.
Vista is not a bad way to go, with the right computer it prevents you from screwing stuff up, it stops viruses and spyware and it comes with pretty much everything you need.