There's sort of a faulty logic to buying a mac because it won't get viruses. It is unlikely to get a virus because people don't bother writing viruses for a computer that doesn't have that many people using it comparatively.
So... What happens when someone decides that they want to write a virus for macs - because it does target a section of the population that are pretty smug about being safe from viruses while not actually taking much in the way of precautions against them.
Is your mac computer actually safe in that case? Anti-virus programs are actually pretty limited for macs... because.... noone worries about viruses for macs. So as the market share for macs goes up... A lot of people are pretty much going to be an open target.
I've been using computers, downloading files/music/or whatever off the internet and participating in all sorts of virus risky activities on the internet for years that I bet a lot of you guys don't... Yet, I've never ever had a virus. I've never gotten spyware, whatever. Because I make certain that my computer is protected with firewalls (both software & hardware), virus scanners, and anti-spyware software and I'm careful what I actually install downloaded stuff on before using it.
My pc is also upgradeable. I haven't "bought" a complete computer in... Okay, I've never bought a complete desktop & I've bought one laptop. I've always bought parts and either upgraded or built from scratch. This is where my comment about just using windows from one computer & putting it on the other. I've never had a problem doing that - whether it's a fresh installation or just popping the harddrive over. You may be legally required to call microsoft (but I never have). I know certain versions of windows have more protection against just taking it from one computer to the next - but I haven't run into it with XP.
But anyways, the point of that is - I can spend $500 on "IBM" parts and build a damned fast computer - and then if I want to put a couple hundred into it over the years - it'll keep up to date. Or you can spend $500-1000 on a nice put together computer (they've gotten cheap) and again, putting some money into it will upgrade it over the years ... Or you can spend $2k on a complete mac, and you're really not going to ever be able to just pop in a new processor or motherboard, so if you want to upgrade that computer to another mac, you can pay another 2k.
As for Vista - it's unstable, bloated, and buggy, you need to have a very fast computer to have it run well... And most people don't get enough benefit from vista to warrant the amount of resources it taxes. (some people do actually do stuff that vista does benefit them) I have a friend who bought a brand new computer with vista, the system crashed constantly... reinstalled vista... same issues. wiped it & installed xp... stable system.
I'm not going to claim to be a huge fan of xp, either... But it's relatively stable and definitely doesn't use as many resources as vista.