M.F.
God knows it can be tough to get things done with toddlers on the loose. But to be fair, cooking and computers are pretty darn interesting to a little person...
So you might try setting aside thirty minutes or so a day, when you aren't too busy making dinner or doing other things. Ask him to play quietly (steering him towards activities that won't become frustrating for him -- a coloring book vs. a blank sheet of paper, for instance). Meanwhile, sit in the same room and read a book. He'll be able to see you, and know that you're present, but don't interact with him. If he tries to engage you, tell him firmly "Mommy's reading right now, this is your own playtime (or this is our quiet time)." Knowing that you're right there might help him adjust, and as a bonus he'll see you reading for pleasure, which is likely to encourage his own interest in books and reading. Some calming music in the background is sometimes helpful too.
You might want to start the same routine with your 18-month-old, if you haven't already. I think, like most things, the earlier you ingrain the habit, the easier it is, and the sooner he'll develop confidence in his ability to entertain himself (which is one of the most important things you'll ever teach your children).
I knew that my daughter would be an only child, and therefore it would be crucial for her to learn to play alone, so I started doing this when she was about one. Just setting up a blanket and some toys where I could see and hear her, then picking up a book, and insisting that she play by herself for about an hour every day.
I realize that boys can be more energetic on the whole, so if it's nice out, maybe take him outside and let run around while you read.
Once he gets more comfortable playing without anyone's help, you might be able to get more accomplished while he's at it.
And if you absolutely have to get things done, you can always resort to a video. It doesn't have to be a cartoon. Most kids love nature shows (Planet Earth series, etc). If his TV time is limited to the times when you're cooking or working on something, he'll probably be thrilled -- and quiet. Best luck!