I found this in a few different places around the 'net - hopefully it's helpful:

Update Computer Clock Through a Firewall

If your computer is a member of a domain, your computer clock is probably synchronized automatically by a network time server. If your computer is not a member of a domain, you can synchronize your computer clock with an Internet time server.

However, the Internet Time service in its default state does not work with Internet Connection Firewall enabled on a dial-up connection in Windows XP.

• Click Start, point to Connect to, and then click Show all connections.
• Right-click your dial-up account, and then click Properties.
• On the Advanced tab, click Settings.
• Click the Add button, and type Internet Time Service for the description.
• Enter 127.0.0.1 for the IP address, and enter 123 for both the External and Internal port numbers, and then make sure that TCP is selected.
• Click OK three times until the Properties dialog box closes.
• If you?re online, a message box tells you that you need to disconnect and reconnect for the settings to take effect. When you double-click the clock on the taskbar, and then click Update Now on the Internet Time tab, your clock will be synchronized with an Internet time server.