Mine goes back 4 months. System Restore can only hold so much data, and is limited by the percentage of your hard drive it is set to use. Mine has 12% of 80 gigs (x2) to work with, it sounds to me yours has less to work with, so you have fewer restore points.
I hope someone else here can tell you how to get rid of Rebate Nation. It may be unlikely, though, unless someone here is willing to put it on their machine to find out how to remove it.
The only other advice I can give you is this:
Go to:
http://www.rebatenation.com
Pick 'Contact Us', and fill out the little form, ask them to tell you how to remove it. If they will not tell you, then contact every merchant they deal with, (Target, Dell, etc),and complain about RebateNation's practices. My bet, though, is that they'll tell you. They are working with some big name companies there, and big name companies are not going to want slimeware outfits affiliated with them, so give it a shot, and let us know how you fare.
Best of luck to you.
edit: At the time of this writing, RebateNation's website seems to be down. However it was working yesterday, so it should come back soon.