Computer Viruses

I have had problems with computer viruses before. Most of them are harmless, if just slightly annoying. But sometimes i pick up some extreme ones. As said in an earlier post, my desktop once had all its data slowly erased until we couldn't connect to and anything and all we could do was turn the computer off an reset it. My first laptop had a virus that caused the  hard-drive to slowly fill up with one gigabyte of data every hour or so. That was fixed with a Disk cleanup. The computer I am using now once had a complete hard-drive crash and had to get a new one. I had the computer for barely 6 weeks. My mom keeps picking up some virus  mostly because she thinks the scans will find them before they do damage.