Flashing an autoloader is safe, but only if you prepare correctly. Do not skip these steps.
Search for credible sources. XDA Developers forums and CrackBerry (now part of Mobile Nations) are the safest archives. Look for the latest available OS for the STL100-2, typically (the final official release). Avoid sketchy file-hosting sites that bundle malware with the .exe. blackberry stl100-2 autoloader