Not quite as simple as that Steiner.
If a device is rated at 100W then it uses 100W every instant (if on full load) it is on and not a 100W every hour as you state.
However most desktop PCs have a PSU (power supply unit) rated between 200 - 300 Watts and depending on ancillaries, such as, Hard drives/Lan cards/Graphic cards/Firewire cards/Ram/USB devices etc. and very importantly processor and processor loads the power usage fluctuates greatly.
What is generally written on the back of PCs is the maximum rated power output of its PSU.
Without knowing a lot more about the OP's setup it is not possible to calculate the power usage with any degree of useful accuracy.
Modern Gaming rigs, if playing MW3, Skyrim on high settings etc, will have +500W power supplies, the upside is most of the power is converted to heat, not so good for the machine, but good for your toes
If your gaming is only pulling 100W, then you're just playing browser games, facebook games etc.