Try to shutdown unnecessary background processes to free up RAM and CPU cycles.
You can download gamebooster3 software to do this.
Game Booster - Download.com
You can try to increase the size of your pagefile.
Change the size of virtual memory (Page file size) in Vista | Windows Reference
I guess you could go all the way up to custom set pagefile size to minimum and maximum of 3072 Mb .
You are up against the physical limits with your 3GB of RAM, it's essential to shutdown anything like skype, AIM, windowscamera programs, cell phone sync programs, etc in the background to try to free up RAM.
Gamebooster will do most of this automatically, you can do it manually, and here is how:
Here is a guide to the windows services running by default in Vista SP2, SP1, etc.
PLEASE READ THOROUGHLY before you start disabling services:
Windows Vista | Black Viper | www.blackviper.com