
Thanks
If you can use OS X, you will probably find Windows 7 fairly easy, it borrows a lot of concepts from OS X - the task bar now acts like the dock, allowing you to "pin" regularly used applications and shows running applications as well. It also has its version of Exposé now as well.
I haven't bought a computer with less than 2GB RAM for nearly a decade, which is a usable minimum for Vista as well. I wouldn't want to use XP with less than 2GB these days - we have machines at work with 256MB and 512MB and they can't really run Windows XP SP2 and AV software these days (a virus scan makes the machine unusable for about 2 days!), no way I'd want to install Vista or 7 on them.
IMHO, 2 cores is more important than going over 2GB these days. That said, I wouldn't look at buying a machine with less than 4GB these days. Windows 7 is faster than Vista and can be faster than XP in some situations. That said, for most daily user tasks, Vista and 7 are more efficient than XP, when it comes to allowing the user to actually do what they want.
As to AV software, I'm with Jon, Microsoft Security Essentials is great. It is light and doesn't use much processor power. For somebody who habitually surfs to "bad" sites and downloads every bit of software they come across, it won't be enough, they will need a "proper" AV package, but for the new / casual Windows user or the expert it is great, it just isn't for idiots.