Use Windows backup to backup your entire hard drive first so you won't lose anything and then do a complete new install.
You will be able to recover favorites, address books, and other data, but there's no way to get around re-installing your software.
I'm not sure what you will gain by going back to Home Premium though.
Is it worth all the work to stop using Ultimate?
I don't know of any issues that would make HP more stable or anything like that?
Mike