Okay I have 2 home computers I currently up graded hard drives on both computers each on had windows 7 on them.
My dads computer I installed win7 on new drive with no issues again.
As for my computer I did install win7 but I cant get full authorization. cause it thinks this is a new computer so I have a three or two left till I have to buy a new version it says, which no way I am doing cause I just got win7 a few months ago.
I did not just copy the new HD over cause I was told by a best buy staff its more trouble then its worth so just did a clean install.
Guess the 'Best buy' was the retail version which can be transferred from one machine to another as you upgrade, whereas the OEM version is linked to one machine and has to stay with it... (Unless you get a friendly M'soft agent who'll let you get away with it on Phone Activation!)
Microsoft's license agreement says one computer per key, so the agent you spoke with was not BS'ing you. Even if you got the Windows 7 Home Premium Family Pack, that still applies. In that case, you would have gotten 3 keys, each usable on one computer, not 1 key you can use on 3 computers.