How much electricity does a pc burn all day, compared to a 100 watt light bulb?

depends on the PC. A good portable will use perhaps 60 watts. A loaded PC with a large monitor could use 300 watts.

100 watts for 24 hours for 30 days is 100x24x30 = 72000 watt-hours per month, or 72 kW-hrs per month. At a typical cost of $0.10 per kW-hour, that is a monthly cost of $7.20

So scale that number up for a 300 watt PC.

But scale it down for hours used. No one uses a PC for 24 hours a day. And, they have sleep modes that save a lot of power when the PC is not in use.

.

If it takes 300 watts, then three times.