What is the best Windows OS for iMac/Parallels 6?

I am a first time poster. I already bought Parallels 6 and I already have 2 iMacs. One is early 2009, the other is brand new January 2011. I really want to run only one or two programs, the most important is Quick Books, not Quick Books for Mac obviously or I wouldn't bother with Parallels. So what Windows OS is the most stable? Obviously speed is not an major issue for data entry.

So, should I stick with XP,(ole reliable), or should I buy Vista, (gag), or System 7? I really don't care about the price, I just need a stable platform to run one program. I do not have a choice, it must be Quick Books and it must be Quick Books for a PC, currently 2009 version. I am linking with an office that only uses Windows XP, for now.

If it is System 7, which seems to be the best best going forward, can I run 64 bit in the iMac early 2009 or should I just get the 32 bit? I plan to buy the Professional version of whatever I buy. Stability.

As you can see,(maybe), I don't want to screw up my iMac, the new one, I am trying to give my wife what she wants on her iMac. All she wants to do is use Quick Books and all we want is to be a PC free household. I really don't want to maintain a Dell 9150 just to run Quick Books.