Registering an Eval version of windows 7

I was not aware that there was a eval of 7, just the RC, if you are using the RC, it told you in the beginning that there was no upgrade path for this

There is an evaluation version of Windows 7 Enterprise that Microsoft offered just after the general availability of Windows 7 (not sure if it's still offered or not). If memory serves, this was a 90 day trial version of the RTM build.

I'm not sure if there is a downgrade path from Enterprise to Home Premium.. you may have to do a clean install after backing up your data.

what is Win 7 Enterprise? never heard of it

Yes there was a 90 day Eval of Enterprise, no it cant be upgraded to anyhting else, yes RC can be upgraded as long as your upgrading to a Ultimate version.