If it really, really is a 64-bit version of Windows then your boot settings may be limiting the amount of RAM that Windows will use. Try this:
- Click "Start", type "MSConfig", press <ENTER>
- Click the "Boot" tab
- Click the "Advanced Options..." button
- Make sure the "Maximum Memory" checkbox is UNchecked.
But if you're running a 32-bit version of Windows then the other posters are correct and these instructions won't help.