to make sure XP can see the most amount of memory possible requires a simple file change.

First open up my computer and go to the tools>folder options setting. in there find the entry that says "hide protected operating system files" and uncheck the box
Next browse to the root of the C drive and find boot.ini Open it with any text editor like notepad and look for the line that looks like this

PHP Code:
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect 
At the end of that line put /3GB so it looks like this
PHP Code:
multi(0)disk(0)rdisk(0)partition(1)\WINDOWS="Microsoft Windows XP Professional" /noexecute=optin /fastdetect /3GB 
if you dont have the /noexecute or /fastdetect in there dont worry about it, those settings vary from computer to computer and dont ahve any bearing on this. Now just reboot and as long as your motherboard can detect all 4GB of RAM (watch as it boots up) windows should now be able to use around 3.4-3.7GB of it.