reinstall ubuntu urgent

[Joep L. Blom]

Hi,
I have an urgent request. I have installed xubuntu 11.10 on y wife's 
system. However, something hs corrupted it in such a way that I couldn't 
start it any more. Disk is healthy (tested with disk utility that shows 
SMART give 200 % OK). Memory checked also completely clean. However, all 
kind of read errors occurred. I tried to upgrade to 12.04 and during 
that process the system hung (> 45').
Of course I have /home on a separate partition but I would like to know 
hoe to reinstall 11.10 (and later upgrade to 12.04) from the USB-stick 
that holds the OS. It runs perfectly from the stick but I can choose to 
install it completely anew ( with repartition) or next to the current OS 
or customised where I choose the partition where to install.
/dev/sdb1 (where originally / was mounted) is already renamed to ubuntu 
12.04 LTS by the upgrade process, /dev/sdb5 (first logical partition) is 
the /home partition/ /dev/sdb6 is /var partition and the other 
partitions I am not sure.
System: AMD64; 1GB Mem; 1 TB disk (other disks are not relevant here).
Can anybody tell me what the safest way is to reinstall 11.10?
Joep