Internal pc clocks are not that accurate. With XP however you can set it to link to an internet time server
Double Left Click on your Windows Time
Click on the Internet Time Tab
Click on the small box to the left of "Automatically synchronize with an Internet time server" to put a check mark there.
Select one of the time servers in the server box then click on "Update Now" and wait until you see the message that the time has been successfully synchronized with the server.
Click on Ok at the bottom of the box