Possible non-OS dependent malware

hi !
my syster's PC with win7 installed behaves strangely. When i try to launch an application it runs for a couple of seconds, then it closes suddenly (sometimes right after the launch) and the dialog window with the shut down options appears. Every time when i try to click CANCEL this window appears again. I can't even select an option because the window is very stubborn (it appears and dissapears permanenty). The applications are difficult (almost impossible) to launch. The AVAST free antivirus (up to date) has found a small file with a "high risk" and removed it. The PC behaves in the same manner as before. Windows Defender does not make it because ir runs a long time and somewhere during the check closes. HDD and RAM check didn't report any problems. I have also loaded the optimal configuration into BIOS, but it didn't help. I wanted to backup my syster's personal files on external media using a linux mythbuntu live CD. Surprinsigly , after launching 2 windows of thunar file manager this behaviour appeared again. it closed the windows and opened the shut down dialog window, beeing even more stubborn than that in windows.
Something tells me that this kind of possiblle mallware is non_OS dependent and somehow hardware generated. What is it and what should i do ?

Does it happens in safe mode as well?

it does happen in the safe mode too.

Well, it seems that the keyboard was the problem (it sent data to the PC as if a certain key was pushed). i replaced it and since then (almost 2 months) there are no problems anymore. Anyway, thanks for the advice.