How to synchronize an internal time server with an external source

1. Click Start, point to All Programs, point to Accessories, and then click Command Prompt.
2. Type the following line, where peerlist is a space-separated list of Domain Name System (DNS) names or IP addresses of the appropriate time sources, and then press ENTER:
w32tm /config /syncfromflags:manual /manualpeerlist:peerlist
3. Type w32tm /config /update, and then press ENTER.

More details at http://support.microsoft.com/kb/307897