Such a huge pain. I switched from the Windows 7 Demo and to the Home Premium.
Not here that if you have a friend in the UK you can get this cheaper from there and shipped over (Retail version ) then it is to buy it in New Zealand, even with the exchange rate in favour of the other way around.
Daft.
 
Anyway, I had Modern Warefare 2 and backed up the steam folder. I installed steam and then added all the missing files and with a small update the games work!
Not tried single player to see if has my save games but I hope it does.
With that I think this is one of the great powers of steam. I did not have to re-install the games.
 Great!