Not enough memory on a 4GB PC

Hi, I'm trying to run Windows XP Pro inside Virtual PC 2007. My host OS is Vista Home Premium 64-bit. When I try to start the virtual machine, it gives me an error message that says, "The virtual machine could not be started because there was not enough memory available on the host." I have 4GB of memory installed on the host PC. I don't have InCD, Roxio, or Nero installed, and Apache, Tomcat, and MySQL are all turned off. I've also turned off Aero, and now I'm on the Windows Classic theme, and mostly all processes except core Windows, Firefox, and Virtual PC have been stopped. I'm trying to allocate 256 MB to the virtual machine, but it's still giving me the error even if I set the RAM to 4 MB. I also have two Ubuntu VMs, both of which give me the same error. Also, I can run the exact same VM on a laptop with only 2 GB of RAM installed with no problems.