Creating a Google AdWords account and running a PPC campaign for affiliate programs is actually a tried and true method of making money. I've never done it before, personally, but I know it can be done. The main issue is that you're going to be bidding on keywords and in direct competition with other affiliates who are advertising the exact same product or service.
Google created a policy about a year ago in AdWords that only allows one affiliate per affiliate program to appear per keyword search. This way someone doesn't do a search and every single AdWords advertiser in the right sidebar is just another affiliate for the exact same thing with the exact same landing page.
So not only are you in competition to get onto the results page with other affiliates, but you have to outbid them entirely to get listed.