Here's a Microsoft knowledge base article about it:

http://msdn.microsoft.com/en-us/library/ms791558.aspx

As I understand it, these are the steps (haven't tried it myself yet):

To enable the 3GB switch on Windows Vista™ 

1. Right-click Command Prompt in the Accessories program group of the Start menu. Click Run as Administrator. 
2. At the command prompt, enter "bcdedit /set IncreaseUserVa 3072" 
3. Restart the computer.

To turn off the 3GB switch 

1. Right-click on Command Prompt in the Accessories program group of the Start menu. Click Run as Administrator. 
2. At the command prompt, enter "bcdedit /deletevalue IncreaseUserVa" 
3. Restart the computer.