Note: We strongly recommend to use a 64 bit version of the operating system if memory becomes an issue under a 32bit environment. According to the German computer magazine c't the workarounds below are not going to work anymore with Windows XP SP2 and Windows Vista.
The maximum amount of memory that can be supported on Windows XP Professional (32bit) is 4 GB. (see for details this article by Microsoft: Memory Support and Windows Operating Systems). The virtual address space of processes and applications is still limited to 2 GB (in practice you will not get more than approximately 1.7 GB) unless the application is enabled for a larger address space (GAMS distribution 21.4 or higher) and the XP machine is configured to support the extended address space. A detailed description of the necessary steps is available at the Microsoft TechNet (How to Set the /3GB Startup Switch in Windows).