An update on this issue... I have successfully installed an OLD HD with Windows XP in to a new PC as a primary drive.
I did not intend to do this, after lightning zapped two PCs lately, I bought a cheap Dell via Ebay! You get what you pay for because within a few hours the new Ebay PC's Hard Drive stopped working (just after I installed all the ham software! ). So, just out of interest, I took the kaput HD out and installed an old HD of the same brand. It booted up but asked for a valid product key. I called Microsoft for assistance and they ask for a series of numbers generated by Windows after I input the XP key from my OLD PC. Yes, despite no use for 2 years I still had the case of my old Emachines PC, the same PC whose HD and OS I was now using in a Dell. The Microsoft rep used the numbers I gave him to generate a NEW key. I used the new key and then was able to re-install Windows XP from a HD partition. I then updated to SP3, also need to validate again but the key from the back of the old PC worked. I'm still updating as I type but so far so good. So now my $46 Dell boots up as an Emachine ! I am not sure exactly what I did but it looks like the answer to my original question is that you CAN install a old HD and OS in to a different PC provided you use the product key from your old machine. Here's hoping that no more HD die or PC get zapped for a while, I'm getting tired of installing software repeatedly. Andy K3UK
