To answer this question, you need to know two things:
If you have a normal desktop computer and monitor sitting on your desk, then it probably consumes something around 200 watts. With a bigger monitor, it probably pushes toward 250 watts, but 200 is a good average. A large color TV consumes about the same amount of power.
If you have read How Horsepower Works, you know that 1 horsepower is equal to 746 watts. So a person would have to generate about 0.27 horsepower to power a computer. Assuming that the generator is not 100-percent efficient, this means that a person would have to generate about a third of a horsepower to run a desktop PC.
If you look at the chart on this page, you can see that, unless you are an Olympic athlete, it would be tough for you to generate a third of a horsepower on a bicycle for any substantial length of time. A "normal person" might be able to sustain a third of a horsepower for half an hour before falling off the bike from exhaustion.
The solution to the problem would be to use a laptop computer instead of a desktop PC. Because laptops are designed to run off batteries, they are very efficient. A laptop might consume 15 watts. It would be extremely easy to generate 15 watts (0.02 horsepower) on a bicycle.
How many calories would you burn doing this? To generate 1 watt for an hour, you burn about 0.85 calories. Rounding up, that's about 1 calorie per watt-hour. So you would burn about 15 calories per hour using your bike to power your laptop. At that rate, a single 60-calorie chocolate-chip cookie could power a laptop for four hours!