In 32-bit Windows XP, you are not going to be able to have Windows assign more than 2GB of memory (physical or virtual) to any one process (unless you have the /3GB switch turned on when you load the OS). And, the total amount of memory you can use is 4GB. These are real limits on the 32-bit operating system. So, your 8GB page file is useless.
I suspect that the solution is paging to temporary files on the HD, which results in only 50% CPU usage. There's only one solution (ok - maybe two...) which is use a 64-bit operating system - either Windows or Linux. Unfortunately, unless your computer is 64-bit capable, this is going to mean new hardware.
Check with your local ASD regarding the availability of 64-bit Flotran in Windows - I use CFX, so I wouldn't know.
Sorry, but your hardware/OS is pretty much max-ed out.