Depends if you mean a fake product key or fake installation media. Or both?
If the DVD/USB you used to install is genuine then no worries. Just buy a genuine copy of Windows 7 and enter the new product key.
http://windows.microsoft.com/en-GB/windows/help/genuine/product-key#T1=tab01
If the installation media was not genuine your only option is to back up your files and re-install. Don't even consider any other option.