Did you do it exactly
as stated in the wiki (with necessary changes for your needs of course)?
Have you checked the menu.lst?
Must ask this because this is crazy
When I write this a thought hit me (hard
This is not used these days, but do you have some Boot sector protection/antivirus?
That blocks writing to MBR
Check BIOS and disable it if it is enabled and repeat the repair grub stages.
You should not have to add XP in menu.lst again
If you do not have that enabled it is either a hidden Boot sector protection or a virus in MBR (or very unlikely some special boot sector that can't be handled in normal ways)
All this is said presuming it is not an Apple laptop, because that changes the entire scenario