When you say you purchased a license for windows 7 pro upgrade, did you purchase an Anytime Upgrade license and attempt to unlock features (which is what Anytime Upgrade does)?
What is the exact failure message you get when you attempt this?
What is the download you are doing now, and from where? There is no download with Anytime Upgrade, so you must be downloading the installer which came with a retail XP pro Upgrade you purchased?
If so, then burn the ISO to DVD using ImgBurn at 4x speed with Verify - it never fails. Boot DVD to clean install: Clean Install Windows 7
The DVD will see the previous Windows 7 OS during bootup to allow use of Upgrade version key. Otherwise use the workaround given in Nate's post above.
There is a workaround to force the installer to in-place Upgrade from Starter to another version, but it sounds like you don't really care about saving your installed programs and a clean install is always best, anyway.