Downgrading from professional to home premium

I recently upgraded my laptop which I bought with home premium preinstalled, to professional edition. However, I didn't realize that I already used my upgrade key for my other computer months ago. The upgrade went through regardless and now I'm stuck using an invalid copy of professional that I need to activate within 30 days. I figure I might as well just go back to home premium, but I'm not sure how to do this. Will I automatically go back down to home premium once the 30 days are up? Or do I have to downgrade manually? Will doing system restore to before upgrading work? Thanks in advance!

 

Did you make recovery disc when you bought it? Does it have a "D" partition which should be the original install of Home Premium?

 

I don't have a recovery disc but I do have a D partition. How do I go about using it to get back to home premium?

 

Who is the manufacturer of the machine.

 

Hewlett Packard. Sorry I forgot to mention.

 

How to do a HP recovery. http://h10010.www1.hp.com/ewfrf/wc/document?cc=us&lc=en&dlc=en&docname=c01443342

 

Thanks, I'll try it and see what happens.