reformatting is not bad. As BigNate said as long as you dont mind reinstalling all of your apps and that you are fine by reformatting. What i would suggest is to get either Norton Ghost or some other imaging software. That way you can reformat. Install what you want to have on there at all times. Take a image. That way when your system gets slow or bogged down you can just put the good imnage back up. Takes away some of the hassle of having to reinstall Windows and all your apps.
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Phone: LG Optimus G Pro
Running: Stock JB from LG with Nova Launcher