Grub not installing when Xubuntu installs

[Bill Stanley]

I am cleaning up an older computer that had before successfully dual 
booted.  I did a full reinstall of Windows and installed all of the 
patches etc.  it is now at Service Pack 3.  Because it is an older 
computer, I installed Xububtu 12.04 (32 bit).  The Xubuntu install went 
smoothly until it came time to reboot.  Grub is missing and the computer 
boots to Windows, (no boot menu).  What must be done to force grub to be 
installed?  On the Xubuntu install disk there is no obvious way to 
install GRUB on an existing Xubuntu install that doesn't have Grub to 
dual boot.  I probably am missing something.

Some information that might be useful.

1. Windows XP SP3 is on a 30 GB hard drive, /dev/sda.  This was 
reinstalled first and no apps have yet been reinstalled.

2. Xubuntu 12.04 is on a separate 120 GB hard drive, /dev/sdb.  When 
doing the install I chose "something else" so I could repartition the 
slave hard drive (/dev/sdb) as such;

partition 1 - primary partition - 50 GB - Ext4 - mounted as /home
partition 2 - primary partition - 10 GB - swap area
partition 3 - primary partition - 50 BG - Ext4 - mounted as /
the rest of /dev/sdb is unused and reserved for future use.

Bill Stanley