I would definitely say "some version of WinXP" because that's what all the support is for now--but I doubt you really need Pro, or even 64-bit. You can probably just get XP Home.
-----People commonly assume there's some huge advantage to Pro but honestly, if you don't know what the differences are already, it's pretty unlikely you'll ever need to use the extra features of XP Pro, and ....XP Home costs less. Online, legit OEM copies of Home are around $82 and Pro or x64 cost around $130, so it's not a lot of money, but it's very-probably $50 that you don't need to spend here.
-----WinXP-64 isn't going to be "necessary" for a long long time either, and the first programs that can really take advantage of it are going to be games, and that won't matter unless your hardware is also updated to cutting-edge (at that future time) as well. So in other words, 64-bit doesn't really matter either. It is true that most CPU's are 64-bit capable, but the main advantage of 64-bits isn't necessary now for other reasons anyway. At this point, "64-bit" is simply a salesmanship feature, if you buy a PC and run Windows or Linux (or Mac! tee hee), or if you buy a 64-bit Mac. ....The processor manufacturers ran a race to produce 64-bit CPU chips, but it's going to be a LONG time before you find a commercial program that you can't run because it only works on a 64-bit CPU.
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