Well i got a pretty old computer. It had 2 worms once.. and my dad got it to his friend to fix it. When he fixed it he said he wouldnt fix it again.. so now its broken again and i really want to remove viruses without paying..heres some problems

1.SO MANY VIRUSES AND STUFF.
2.WHEN I OPEN A PROGRAM IT ASKS ME WHAT PROGRAM I WANT TO OPEN IT WITH.
3.I HAVE AVG 8 BUT SINCE I CANT OPEN PROGRAM SINCE IT ASKS ME WHAT PROGRAM TO..I DONT KNOW WHAT.
4.JUST MESSED UP I TRUELY WISH TO FIX.

I REALLY WANT TO FIX!!! SO ANY SOLLUTION LIKE A GUIDE TELL ME.
Here my questions:

1.I want to get a fresh new start i got 2 cds..the 1st one says:
DRIVERS AND UTILTIES- ALREADY INSTALLED ON YOUR COMPUTER-DVD for Reinstalling Dell Inspiron 530/530s Computer Software-THe software is already installed on your computer.Use the media only to reinstall the software-Contents Device drivers Diagnostics and utilities-I got windows Xp!