http://knowledge.autodesk.com/support/autocad/troubleshooting/caas/sfdcarticles/sfdcarticles/How-to-enable-a-3GB-switch-on-Windows-Vista-Windows-7-or-Windows-XP-s.html
To enable the 3GB switch on Windows Vista™ or Windows 7:
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 disable 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.
