I have an occasional need for an MS Windows machine (a few times a year). My main machine is a Mac, and I'd like to get a cheap and legal MS Windows (XP or later) virtual machine that I can run on it. I have VirtualBox installed already, and I don't want to use Wine or something like it.
If the cheapest way is to buy Windows XP CDs on eBay, I would appreciate clarification as to exactly which variant I need to buy as prices range from $10 to $260 and I find it all very confusing. Is any version with a COA ok? or does it need to explicitly say non-OEM, retail version, etc.?