if his PC is hooked up to the internet try websites like cbeebies or cartoonnetwork for simple on-line games. my 5-yr-old's PC is running Windows95 and he spends hours playing these games. some of them run a bit slow, but thats no bad thing if he's still learning to use the mouse/keyboard - and watch out, soem of them can be quite addictive.

if he's not on the internet, then beg/borrow anything from friends - eveyone has got old games lying around at home. as a rule of thumb, anything which comes on floppydisks instead of a cd should run just fine. my boy likes the old arcade games like space invaders, as well as the educational kids games, or even the really old 'brickout' or 'pong' games - I downladed some from the net for my daughter several years ago and copied them onto her PC - can't remember the site name though.

get him a joystick and an old copy of microsoft flight simulator, give him 1/2 hour of 'flying' tuition and take bets on how long it is before he discovers that crashing into tall buildings is great fun....took my lad about 2 hours!