your only gonna get a virus if your pretty st00pid.
Which means - no file sharing software!! no crappy plugins, no accepting any file(within reason) etc.
Ive operated on PC ( :/ ) for about 9 years and only had 1 virus when I was downloading off kazaa sometime ago which chruned all my media files into visual basic.
As for someone hacking your machine, tis very unlikely, but never the less if you feel the need there is ample firewalls out there.
Also you can give your machine a free online scan for viruses as they usually update their web stuff often so it will detect all the newest exploits etc. Since nowadays it isnt safe to even click a .jpg link