IF you are refering to the built in clock on your motherboard, they are well know for being wildly inaccurate. The 'bit' they use to 'count time' is really rather cheap, and would compaire to the £1.99 watch you could buy from 'Del Boy' down the local market. After all, the manker of the board want to make a computer, not a clock.
There are a number of downloadable utilities that will automatically check the 'real' time and adjust your computer if it's wrong. Win XP has such a thing built in.