Desktop computer switched on for 8 hours

CO2e: 0.58 Kg (10 hours)

How do I find out how much electricity something uses? says

140-330 watts, Desktop Computer & 17″ CRT monitor

Low power consumption PC’s:
“Asus, Build your own AMD Power Saving PC
Typical desktop computer uses about 85 to 250 watts. Yes, some computers
use more. Our power saving system consumes 55 watts on idle. In doing so
you are not sacrificing performance for typical home & small office activities – email,
web browsing & general office applications.”

A rough estimate might be 200 watts.

For CO2 from electricity we have 0.1 Kg per megajoule

Assume large Computer is 200 watts.  8 hours is 28800 seconds.
Electrical energy is 200 * 28800 joules or 5.76 megajoules.
This creates 0.58 Kg CO2.

Michael Bluejay also does specific computers:

Configure my PC to Save Electricity – Queen’s University Belfast

Why estimate when you can measure?