You could try to download a Bitcoin miner (such as GUIMiner) and see how many hashes per second can your PC mine.
However, right now is quite difficult to earn profits as a particular with a common PC, because the difficulties are really high and the relationship earnings/expenses will probably make you loose money every month, because of the power, hardware and maintenance costs.
There are websites (like this one) that makes an estimation of the profits (or losses) you can get, by entering some basic parameters, like how much do you pay per kWh and how many hashes are you able to mine per second.