laptop trouble

Hi all:

My laptop used to be fine, ok until I stupidly connected it to the net - without any protection or firewal..

So a million viruses had a fun time deleting all my PDF files. I sent it to the computer guy and he fixed it up - its ... not so nice and fine anymore.

But one major problem I have is keeping the display settings. I turn on the laptop and it comes up with the display settings I have chosen (so appearance, wall paper etc). Then, lets say 5-10 mins later, it makes a "Beep" sound and the next thing you know, it changes - the wallpaper is changed to a "none" setting and all the alarm settings I had are cancelled.

What on earth??

Thanks for any advice.