fix mbr

Hi all
I had to do a repair in XP and of course that has wiped the Ubuntu mbr, I can not get into Ubuntu. no boot sector. I have tried some helps on line to fix it, but nothing worked. maybe help is for older versions of Ubuntu not sure, or it me :-) !!!!
I am running 10.04
sda1 = ntfs (Win Xp )
sda2 = extended
sda5 = ntfs (shared HDD )
sda6 = ext4 (Ubuntu)
sda7 = linux-swap

There is no sda3 that I can see is that normal?
please can I get some help to fix the mbr so I have Ubuntu back. I don' want to reinstall .
O Please make it simple to follow ;-) I get a bit confused when it is very technical....
  Thanks
Neil