infected vista computer _ spyware everything is screwed up please help?

I'm on a site a number of days that was safe <but there was a link of the existence of a Web site that I thought would be cool if I clicked on a link to verify, but it 's is proved to be a porn site and now my computer keeps things close and little messages popping up to say that I have a door Trojans and my speed Internet has dropped from thirty-nine% and everything> and as you can see my keyboard is incoherent because it won 't let me put it in the proper characters>> I anti_spyware software, but it is not fully compatible with Vista and i haven' t got the money for new things> so I was wondering if anyone knows of some free scan and removal software that works with Vista and they know it can fix my computer or if I just need to reformat my computer> if I do reformat I was hoping maybe for some detailed instructions As I have very little knowledge of computers> I herd that I need my backup CD if I need to reformat>>? thank you