i highly recommend you to go maxkrill.com. where you can find very affordable and cheap product key of windows 7 and other software. yesterday was i bought windows 7 professional from their site. and it worked very perfect and its permanent.
they have very good customer service. i got my key in 10 mins(email).I have already recommended this site to many friends.so they can save some money and time both. if u want to compare with others too. you can compare before buy. it is very good for you.