If the cat only does it when you are actually using the computer, its probably just being affectionate towards you, trying to get your attention / interact with you

(this is a good thing! however untimely hehe)
Pop into whatevers nearest (pet store etc) and shell out a couple quid on some cat toys (the more durable the better). A little stimulation goes a long way
One of ours has a distinct affinity for following the pointer... wether its the mouse cursor on a computer screen or just a red dot on a wall, or even shadows, including his own
its a year old and unlike our previous pets does not seem to have outgrown it
All sorts of fun if you set your computer up with a projector against a large wall :P
Be happy it does not have a "possessive" personality... literally all sorts of everyday objects he will pick up with his mouth and toddle away to dump in his cat bed with the toys... including house keys which has caused all sorts of panic!