the easiest thing to try if its only happened just happened recently and you can remember a date when all wa well is to use system restore if you have windows xp.
try clicking the start button then click 'all programs/accessories/system tools/system restore'
choose 'restore my computer to an earlier time' and look at the calender, choose a date when all was well and click next. restart and let it restore. if it doesn't solve it you can easily undo this by doing the above again and click 'undo my last' you wont lose any data by doing this so its worth a try.
failing that if you can find out what motherboard you have then let us know the make and model no. please re installing drivers may help if system restore doesn't help.
if you don't know try downloading everest home edition and install and run it 'start/all programs/lavalys/everest home edition' then click 'computer/summary' and look for 'motherboard name' and 'audio adaptor'