10. He's distracted by cats chasing his mouse.
9. SIT and STAY were hard enough; CUT and PASTE are out of the question.
8. Saliva-coated CD's refuse to work.
7. Three words: carpal paw syndrome.
6. Involuntary tail wagging is a dead give-away that he's browsing www.purina.com instead of working.
5. The fire hydrant icon is simply too frustrating.
4. He can't help attacking the screen when he hears "You've Got Mail".
3. It's too messy to "mark" every Web site he visits.
2. The FETCH command isn't available on all platforms.
1. He can't stick his head out of Windows XP.
Joke copyright unknown.
Thank you Ritesh G. for emailing it.
UPDATE: My Twitter Minion, LoupiCat tells me the real reason the dog can't use the computer is because the cat hasn't finished with it yet!