I am going to assume that you are talking about the Vostro in your signature... It has this CHIPSET
which according to Intel specs should support up to 8GB of memory addressing. Then I look it up on Dell's website HERE
and if you read the footnote at the bottom about the RAM...
1 Desktops configured with 4GB of Memory or More: The total amount of available memory will be less than 4GB. The amount less depends on the actual system configuration.
In all honesty I don't know why that would be, unless of course this is just a disclaimer for people trying this with a 32bit OS. Check in the system BIOS... see how much memory it says you have installed. Like my E1705 notebook has the capability to hold 4GB of RAM also, but the chipset will only address 3.25GB of it - and this is reflected in the BIOS screen. Find out what yours says and go from there. Perhaps there is an issue with x64 XP and this chipset... I would assume you downloaded and installed the proper chipset drivers for it from Dell right?