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#31
You will get and keep a perfect reinstall (even for retail) to the exact extent you use the tools and methods in Clean Reinstall - Factory OEM Windows 7, which compiles all of what's worked best here helping with tens of thousands of Win7 installs.
The way the slide into ruinous infection begins is allowing freeloaders to sneak in during freeware installs like for flashplayer. These can lodge in browser add-ons so don't allow anything there except Flash Player, nor any search except Google in the stable browser search box which prevents spying except in Chrome which is itself spyware.
Just uninstalling these intruders isn't enough as you need to root them out of the registry where they can keep running after uninstall. For this use SUPERAntiSpyware on demand scanner. As with everything else except your AV, keep this turned off in msconfig>Startup and >Services after Hiding all MS Services.