My advice from personal experience is the following:
- Don't use Windows XP if you can possibly avoid it, unless you do next to nothing with your computer. If you don't like the naff shiny bits in windows 7, turn them off.
- Only run one malware defeating product, and if you must use more than one preferably have them from the same vendor, otherwise they tend to overlap and fight each other, slowing you down massively.
- I personally use Windows 7 with Microsoft Security Essentials and the default Windows firewall, and I deployed it on 20 computers at work (some Vista, most 7), and we (and I) have had zero virus-related problems since I did that.