If you go into Device Manger (XP) - Start-Right click on My Computer- Properties- Hardware - Device Manger. Is there any warnings or drivers that dont look like they are install correclty? If everything is good with the sound there, Go to Start - Control Panel - Sound & Audio Device, now check to see if your computer has picked out a driver for your sound to use (you can do a number of tests to see how it is setup).
Silly question, what colour port did you plug your speakers into? Have you tested these speakers on some other device? If you know the Mainboard Name and number you should go find the latest update. If you dont have a clue about the mainboard and dont want to go open it up. download Belarc Advisors and it will inform you about everything in your PC e.g. Mainboard Maker and number. http://www.belarc.com/free_download.html