Well that's how Spyware happens.  Spyware installs itself as a program, in the Windows registry, without you ever actually clicking "Install" or "Run."  And it "hacks" at the registry, to accomplish this, leaving an unstable one behind.

How often you need to reformat your drives  depends on tow things.  First, where you are surfing.  Ans Second, how much security you have present when surfing there.  The safest thing to so is set your IE cookie preferences to not accept any cookies, and purchase AntiSpyware, AntiVirus, Firewall, etc. protection.  The only problem is you'll find many sites not working.  So, in this case you'll need to Allow cookies, on a site to site basis.  Conversely, if you have not purchased protection, nor have you increase your Cookie Security, and then surf through 100 sites everytime you see an add to Punch the Monkey for a free PSP, or go if yuo then/also go galavanting through adult content.., then your system's life expectancy becomes significantly shorter,