Crashing Mac - help!!

[lostkat]

My Dad is having issues with his Mac (don't ask what sort, it's a huge white thing with the computer/screen integrated into 1 unit). He says that it's fine in the mornings, but around lunchtime it starts freezing on him, getting black lines across the screen etc. He can't force quit the programme so he has to just turn it off and back on again.

He's a photographer, so is mostly using Photoshop.

He tried to re-create the situation for me yesterday, worked on an image for 5 mins and all of a sudden Firefox windows started popping up. There were hundreds of them and they just kept coming up and he couldn't do anything to stop them. I asked if he'd swapped the keyboard in case it was a key getting stuck down and he has.

I know absolutely bog all about Macs, so I'm at a loss as to what to suggest. It looks to me like a virus, but I know that Macs don't really get them.

Any suggestions???