If you install the trial version of Expression Web 2 then purchase a license of it you can enter your product key in by following these steps.

1.  Within the control panel open the "add/remove programs" or "programs and features" if you are using Vista.

2.  Select "Microsoft Expression Web 2" in the list then click "Change"

3.  You will see an option "convert" select this and you will be prompted to enter your product key.