I need help with my infected computer.

I recently went to install Norton 360 but cannot because the norton installer is unable to authorize my microsoft windows installer. I've tried everything from reinstalling microsoft windows installer, to the fixit tool they have to no avail. I think this is happening because a few days ago I downloaded something I knew i shouldnt have and am now infected with all kinds of malware and viruses. I went to the symantec website for help and they want me to pay just about 100 bucks to get my computer clear(after paying 50 for the program which i cant even use,go figure). I think I'd like to see what I can do first, before i shell out my money. So where shoud I start? What do I need to do? I'd like to thank whoever replies in advance for your time and help. Thank You.

Hi, moe2the. If you think your computer is infected, please refer to this post, and follow the instructions, beginning with Step 3. We will certainly see what we can do to help you.