*how often should you defrag your hard drive and is it an important process that should be done???*
About every 3 months or so. IMHO it's highly overhyped as a measure. I tend to do it after I have a big bout of software try outs (installs/uninstalls) - but otherwise hardly ever. I have never noticed any performance difference before and after deragging, I just regard it as a *general tidy-up*
*is a trojan horse a virus??*
Yes - just a particular type - what's in a name anyway - virus, trojan, worm, spyware etc etc - they're all bad.