In my opinion, I think Avast and Norton are good antivirus/internet security programs. I use Norton 360 Premier Edition alongside with Malwarebytes' Anti-Malware PRO. I used avast in the past and it was pretty good, but you need another program running with it, like Malwarebytes' Anti-Malware, to get good results.
Edit: Also, if you want your computer to be fast as the day you got it, you can always do a Factory Restore. It deletes ALL THE DATA on your computer, and resets your Windows to default settings, so make sure to backup the important files on your PC onto a USB, an External Hard Drive, or some other device that you can save the files on. Also, I think the drivers stay as they are because my cousin did a Factory Restore on his Vista Home Premium x64 OEM, and all his drivers were already installed and working right after he did that. The only thing that changed was all his files were deleted, and his Windows Vista was in the condition as he got it on the first day.