Quote from
http://www.tipandtrick.net/2008/how-to-enable-pae-in-x86-32-bit-windows-server-2003-and-2000-to-use-large-4gb-or-more-ram-memory/"To enable PAE in Windows Server 2003 and Windows 2000 (and Windows XP), append the /PAE switch to the end of the line of operating system in the Boot.ini file. To disable PAE, use the /NOPAE switch. The Boot.ini normally located in the root folder (i.e. C:\) with Read-Only and Hidden attributes, which are required to be removed before the file can be edited.
For example,
Original Boot.ini:
[boot loader]
timeout=0
default=multi(0)disk(0)rdisk(0)partition(1)\WINNT
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINNT=”Microsoft Windows 2000 Advanced Server”
/fastdetect
Change the Boot.ini to become:
[boot loader]
timeout=0
default=multi(0)disk(0)rdisk(0)partition(1)\WINNT
[operating systems]
multi(0)disk(0)rdisk(0)partition(1)\WINNT=”Microsoft Windows 2000 Advanced Server”
/fastdetect /PAE
For Bootcfg command, use:
bootcfg /raw "/pae" /A /ID 1
Restart the computer after modification to make the change effective.
Note that even with PAE enabled, there is still memory limit imposed by the operating system itself. The table below list the maximum memory support with PAE enabled for Windows 2000, Windows Server 2003 and Windows XP. Most 32-bit client based Windows operating system, such as Windows XP and Windows Vista (see way to use large memory of more than 4GB in 32-bit Windows Vista), has OS level limit that restricts the maximum amount of memory able to be used to 4GB."
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In your boot.ini file on the last line where it says "/fastdetect" you would modify it to look like this "/fastdetect /PAE" with out the quotes. You should be able to right click the boot.ini file and open and edit with notepad. Dont forget to remove "read only attributes" to be able to modify the boot.ini file.
I dont know if it will work, as I have never done it before. Like I said I was only able to see all my 4gb ram after my video card and sound card died, thus freeing up space in the "physical address space".
Keep in mind that this "boot.ini" file is what your computer needs to boot into windows properly, so if that gets messed up you might have a hard time booting. I have personally messed up my boot file on vista many times trying to get linux to work in a dual boot environment and had to learn the hard way on how to fix it. So if something goes wrong then you should be able to boot from your Windows xp disc and repair it.
It could also be a faulty ram module like George (aqr) said, so double check to see if you bios can see all 4 gigs of ram first. If your bios doesn't see all 4 gigs, then that either means a bad ram module or an improperly seated ram module.
You can access the bios by pressing f8 or f10 repeatedly during boot process as soon as the computer is powering on.
Hope this helps, and let me know how it went.
G.Driggs