Microsoft has changed the way the "3G Switch" is set in Windows 7 and Vista.
Start, Programs, Accessories, Command Prompt (right-click the Command Prompt icon and select Run as Administrator)
In the Command Prompt enter:
C:\>bcdedit /set increaseuserva 3072
The 3072 is based on how much RAM you have, see note below.
Specifies the amount of memory, in megabytes, for user-mode virtual address space. This variable can have any value between 2048 (2 GB) and 3072 (3 GB) megabytes in decimal notation. Windows uses the remaining address space (4 GB minus the specified amount) as its kernel-mode address space.