You should be following the steps in Clean Reinstall - Factory OEM Windows 7
to get a perfect install.
It explains that as long as you are certain you installed the licensed version on the COA sticker, then the problem with key rejection is almost always an incorrect character. B's look like 8's. Triple-check your input.
If you own Basic then you had to unlock all versions using ei.cfg tool. Did you then get a menu to choose Basic during install?