Where did you obtain your copy of W7 ? If you bought it from a risky source and almost certainly if you just bought "just a key" then it's a good probability it is a bad key that has been sold to so many people that Microsoft has shut it down.
If you bought it on eBay then file a complaint immediately. If it is beyond the 45 days then hopefully you used a credit card because you will have to try to take it up with your credit card company.
If this is not the case then use the phone number to call Microsoft and
activate by phone.
EDIT: Even IT specialists can get snookered by these scam artists because sometimes it is very difficult to tell the fakes from the real thing. Some software counterfeiters are very good, holographic images and everything. Fell for it myself because it worked for 4 months before it shut down, too late for me to do anything about it. Contact the installer and ask him the source, he should stand behind his installation and remedy the problem (ate the cost out of my own pocket rather than potentially loose a client).
