Problem with my computer, assuming it's a virus

It's a Windows Vista, around six years old, and we have had various problems before with it, yet none that were as bad as this.

I am assuming the problem is a virus.

It all started around two months ago, where the mouse (USB connected) would stop working occasionally. Unplugging and re-plugging it would only make it work again for a short while, whereas i could plug a graphics tablet in, and it would work. This problem only worsened over time, to the point that all USB ports would simply not work. A message would appear on the toolbar saying that hardware had malfunctioned, yet there was nothing that could be done to stop this. My dad then opened up the computer and connected another USB drive, which worked for a short while, then the computer would just not start up, it would go to the blue screen, or say there was a hardware problem. Removing these USB drives would make the computer start up fine again, but only for a short while, as the same problem occurred, which was only resolved when some of the computers memory was taken out, yet after some short time, the same problem occurred again. The computer could be started in safe mode however. Due to my Dad's messing with the computer, I am not sure of anymore physical hardware alterations at the present moment.

Presently, the computer is switched on normally (I am not sure how).
Our current main antivirus software is Avast. We used to use AVG, yet this only seemed to make the computer lag and run stupidly slow. We also have malwarebytes installed, yet this does not even open in the computers current state.
For some reason, the computer won't connect to the internet (although plugged in via cable). Avast has also been disabled, and will not restart.

It is these most recent problems which are telling me that the problem may be a virus. We would be willing to completely reset the computer, however there are files (pictures, videos etc.) which we would really want to retrieve off the computer (yet with no working USB drives, this is a problem)

Any advice or help on the situation would be greatly appreciated.
Thanks
 
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