Google Calendar Sync
Synchronise your Google and Outlook Calendars. You can opt for a one way sync (either direction) or a two way sync. Currently, Google Calendar Sync is only compatible with Microsoft Outlook versions 2003 and 2007, and operating systems Windows XP and Windows Vista.
1. Download Google Calendar Sync.
2. Enter your Google account details
3. Select your sync options
4. Hit the “Save”
Google Calendar Sync options
Google Calendar Sync in System Tray
Issues?
I am running Outlook 2003 SP3, which is a supported version, but unfortunately Outlook continually locks up with a MS LDAP error;
Unable to display results because too many entries in the Internet Directory match your search criteria.

Some quick Googling and we can find a registry edit that should help;
MS KB 311829 FIX: LDAP search error when you try to use a full name to search the Display Name field {Outlook 2002}
MS KB 325314 Error messages when you search the address book by using a full user name {Outlook 2003, Outlook 2002}
In short a new key and DWORD are added to the registry and the issue appears to go away.
[HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\ldap]
"NoDisplayNameSearch"=dword:00000001
[1] Calendar Help: Introduction to Google Calendar Sync [Google]
[2] Calendar Help: Sync With Microsoft Outlook [Google]
[3] Calendar Help: Troubleshooting Issues with Google Calendar Sync [Google]
[4] Calendar Help: Google Calendar Known Issues [Google]
[5] Google Help: Google Calendar Sync – FAQ [Google]