Can't read 4GB memory

My computer is installed 4GB ram, it's running on Windows XP pro SP3(32bit version, My computer properties show 3GB physical memory). However, in the photoshop memory usage(Edit>>preference>>performance) show available RAM is 1678MB only.

What's problem occur? How can I using more memory?

Thanks a lot.

Please check the Microsoft knowledge base on how to set the /3GB switch in your boot.ini. Once done, PS and all other Adobe apps should be able to use up to 3GB on XP.

Mylenium