Why? Who knows... Sure. It could be an infection. Could be bad memory. Could be a failing hard drive. Could be a really messed up installation of Windows. Could even be that the programs weren't installed correctly. Impossible to give you a definitive answer though, based on the information you have supplied.
Just being honest.
You want a solution? Perform a factory recovery based on your computer manufacturer's instructions. If there is nothing wrong with any of your hardware (your Ram is good, hard drive is good, etc.) then this should take care of your issues.
If you want to know if it is an infection, you could always use Anti-Virus tools. There are multiple different suites out there... and your computer most likely came with one.
You could also bring the computer to a computer repair shop, and pay a professional to take care of the problem. I mean, this is a complex electronic device that costs as much as a used car... just because it fits on a desk doesn't mean that everyone and their brother should be able to fix it themselves.