Remote Desktop is disabled by default in Windows Vista, but it's easy enough to turn it back on. If you need to access your Vista PC from another box, it's an essential thing to turn on.
To get to the configuration page, you can either right-click the Computer icon and choose properties, or you can type in system into the start menu search box. It'll be the 4th item in the list, as you can see here:
Now you'll want to click the Remote Settings link on the left hand side:
Now you can finally turn it on:
To connect from another Vista PC on the same network, click the bottom radio button. If you need to connect from an XP/2k machine, click the "Allow connections from computers running any version of Remote Desktop" radio button.
Don't worry about setting up firewall rules, Vista does that for you automatically.
Note: This will not work for Home editions of Windows Vista.