I have XP Home on this P4 (was Win Music Centre Ed' But THAT's another story!) but I had XPro for a long time on another, better specc'ed machine that Son used for his music (tho' this P4 is pretty good) I have another AMD 2.7G 2core machine with Pro on it and 1 desktop and one lappy with W7/64.
I never and HAVE never noticed any difference between Pro and Home (I also have a works lap' with Vista. Now that IS cack!)
Re activating XP: I had a PC built 8 years ago, cost me the thick end of £700. It eventually came time to replace it (I needed a PCIe slot for one thing) and so I bought an Asus MOBO/Proc'/memory deal from Dabs of Ireland. I had the original XP Pro disc that came with the build (the shop had long since gone bust!) but I expected trouble since I have heard of peeps having difficulties with Msoft after just changing a hard drive. All that was left of the original PC was the case and PSU!
But not a bit of it! "They" said "not activated, click here" I did and bish-bosh-bash I was legal!
Yeah, if you are not going 64bits and therefore lotaram (just put 8G in my W7 desktop) XP is cool.
Just seen that I could buy a 1TB 3.5" drive for fifty quid. You might find it easier to simply fit a new, clean drive and put XP on that. Keep the old drive handy because XP will die one day! (or rty the drive in another build?)
Dave.