Problem with Grub installing Xubuntu 12.04

[Joep L. Blom]

I revently got answers toa curious problem I had with the system of my 
wife (See mail:"reinstall ubuntu urgent"). It turned out to be a 
corrupt xubuntu 11.10 kernel (I even replaced a perfectly good 
Sata-cable) mimicking hardware problems.
But the second problem occurs now with Grub. I installed 12.04 over the 
old installation (i.e. I installed over the old /root, /var and /local 
and also /home but none of them were formatted.
OK installation went like a breeze. However. when I wanted to start it 
it first came with the start menu showing all possible installed OSes.
However, when I wanted to start any one of them Grub says:
"error: no such partition" (twice) "Press any key to continue", and when 
I do that it comes again with the start menu.
I can of course run xubuntu from a Live USB-stick and I have installed 
Boot Info Script to see what Grub is using, That looks perfectly normal 
to me. I says:
"Grub2 (v1.99) is installed in the MBR of /dev/sda and looks at sector 1 
of the same hard drive for core.img. core.img is at this location and 
looks for (,msdos1)/boot/grub on this drive."
All other info looks perfectly OK to me. However, I am an ignoramus of 
Grub2 (I could work with the menu.conf of Grub Legacy) but I have no 
idea how GRUB2 works.
I hope someone with knowledge of GRUB2 can help me out. My wife uses 
temporarily again her old Windows XP system and I want to get rid of 
that old & buggy system.
Thanks in advance
Joep