Few month ago I published an article "PXE, aka Pre-Execution Environment - Part 1". In this article, I talked about a problem to use boot disk which created at Windows XP for PXE boot. I'd like to say thanks to Michael Bridge. He found a way to make the Windows XP floppy image of the netbootdisk work.

Under \\pxelinux.cfg\default

append initrd=.IMA raw

(add "raw" after the image name)

According to this http://syslinux.zytor.com/memdisk.php it allows access to protected mode memory. Michael Bridge, told me that he managed to get the solution on this web site:
http://syslinux.zytor.com/archives/2007-July/008918.html 

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