Do you have all of the original disks which came with the computer? If he Frankly, you should do PP's steps first I would think. If the computer is full of malware, etc., then just restoring would not really get you very far, if as you say, he probably has "bad" browsing habits then you probably wouldn't have a good restore point to choose from without "something" lurking there someplace.
Is there a way to hook-up directly to the internet, without going through the router? Is the operating system XP?