I went without an antivirus for 2 years and everything was fine. Then somehow I picked up the WIN.CIH virus and it infected every damn executable on my hard drive. Cleaning it was a mother because it would infect the antivirus program I was using to clean it and then spread to even more files. I had to download a specific program to isolate the virus before cleaning the pc. Formatting wasn't an option because I had some really important programs on there and besides, viruses can infect the bios sometimes too and even a format won't do the trick. Finally got the damn thing cleaned off, been running Norton's Antivirus since and it has saved me many times.