Hi Tony M
Windows XP was a cracking OS, however support for it stops in 2014 so is best now avoided.
Vista was a complete unmitigated disaster for Microsoft, so much so that they re-introduced XP as computer manufactures and customers were refusing to accept Vista, Microsoft also bought forward the development and launch of Win 7 which even in its Beta phases was far superior to Vista. (It also would work on computers that ran XP whereas Vista required you to upgrade to a more powerful computer)
Go with Windows 7 pro, as if there is the odd problem with some of your old programs or hardware, (Pretty small unless it is really old) you can download virtual XP which runs within Win 7 solving any problems.
If you go with XP make sure you set the motherboard Bios to its defaults, as this will set the SATA drives to IDE mode which will allow XP to load without problems.
If you want to use the more advanced features of SATA drives (you will need to set this in the motherboard Bios) press F6 when the XP install asks if you wish to load a Raid or 3rd party driver, then install your motherboard disc and choose the correct SATA driver, then just follow the instructions and you should be fine.
My recommendation however (And I have been working with computers of all types for over 30 years) is most definitely to go with Win 7 Pro.
Hope this helps
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