Answer:
Assuming you mean "how much carbon dioxide is generated by the electricity consumption of a computer", you are actually asking a very complicated question.
In the first instance, you need to know what the energy consumption of your/the average computer is. This depends on a huge number of factors such as: how powerful the processor and graphics cards are, how many hard drives and optical drives it has and what kind of monitor it has and what size that is. Also, the consumption varies depending on whether the computer is under load (performing calculations and reading from or writing to hard drives or optical media). If you're not interested in accuracy, I would guess about 200W for a modest office machine rough average.
Next, it depends on how much CO2 is produced per unit of electricity from your supplier. This in turn depends on what sources are used to provide your energy. Coal and oil produce more CO2 than gas, nuclear and others. The calculation is likely to be very complex and require a lot of research. The answer could be in the region of 500g per KWh, but this could vary wildly depending on which country you are in.
So taking our two assumptions (0.2KW and 500g/KWh), we can very roughly estimate 100g of CO2 produced per hour of computer usage.