I don't think I've ever had a RETAIL ver of any MS OS.....and these days I use TechNet ....for the price of one Retail W7 Ultimate I have 10 licences of 7, the same for Vista...same for XP.... all variations of them....Office vers [all].... and countless other tricky bits.
Jafo, you are allowed to install each Win 7 key on 10 different machines and you are allowed unlimited re-installs on those 10 machines.
If you generated all of the keys when you originally signed up with technet you should have 10 keys of each Win7 products. Microsoft changed this to 5 sometime last year. With 10 keys, you can legally install on 100 machines.
BTW...thats's 10 of each version of Win7...10 Ultimate, 10 Home Premium, ect.
Also, don't forget, you have a Enterprise edition of Win 7 that has unlimited installs on as many machines as you care to put it on.
One last thing, your keys do not expire if you decide not to renew your subscription but you do lose the abitlity to download.