XP Pro has a built-in ntp client. You don't need any special client software.

run gpedit.msc / computer configuration/system/windows time service/Time Providers.

Configure the client, enable the client.

Under configure the client, I have it set to:
Enabled
ntp server: 0.us.pool.ntp.org (use any ntp server you want)
Type: NTP

Then enable the client in the other menu item.

You don't need to enable the server.

This lets you sync more frequently than the item in time and date properties, but that one works also if you don't care about syncing more than once a day.

If your date/time is off by a day, it won't sync automatically.

See also:

http://tf.nist.gov/service/pdf/win2000xp.pdf

(which describes ntp client configuration using the 'net' command, but it's the same thing.)

Also, if you are part of a domain, you want to use the domain's time server, and not an external one.

-David