Computer scrolling up and down

Please can someone help me? My computer seems to sometimes have a life of its own.

The screen scrolls up and down on it's own, sometimes it flicks back through internet pages or document pages that I have been viewing.

other times it starts messages on my yahoo messenger and types garbage or types in sentences which i have used previously. usually the message is typed to 'rrr' or someother random address, it generally does not send messages to my yahoo address book.

other times whilst I am typing, the curzor moves up into another sentance so that i am not typing in the right section. When I move it back down it is ok for a while then moves again.

I am protected by Norton 360 and have run many virus scans but they show nothing.

has anyone got any idea what is wrong?

many thanks.

Check if you have Infrared (irda) enabled, or voice input.