Connecting to the net and installing the AV software are the last two things I would do.
Best bet may be to format and start again - or use whatever recovery disc comes with the Mesh system these days.
Then install Office and continue with any other software. After you have installed something make sure you reboot the computer, even if it doesn't say it is required. By doing that you should be able to see what is the problem.
I very much doubt it is a hardware thing.