That's a bad URL (someone is squatting on it and it's not owned by Microsoft). You want to go to www.windowsmobile.com for Windows Mobile/Pocket PC resources. Here's the URL to download ActiveSync for Windows XP: http://www.microsoft.com/windowsmobile/en-us/help/synchronize/activesync45.mspx .

If you're running Vista then you'll use Sync Center instead, which is an installable component of Vista. Here's the URL for that download:
http://www.microsoft.com/windowsmobile/en-us/help/synchronize/device-center.mspx#download