Try re-installing grub.
Whilst booted with the USB drive - open a terminal and become su, and follow this
You enter this ---------------- grub
Computer returns like this ---- grub>
You enter this ---------------- find /boot/grub/menu.lst
Computer returns like this ---- (hd0,1)
Here, (hd0,1) is Grub's pointer to my openSUSE installation. Your pointer should be the same, I edited this example to (hd0,1).
You enter this ---------------- root (hd0,1)
Computer returns like this ---- Filesystem type is ext2fs, partition type 0x83
You enter this ---------------- setup (hd0)
You see several lines like this --- Checking if /boot/grub/stage1 exists ... yes Computer finally returns this-- Succeeded.......Done
You enter this ---------------- quit
You enter this ---------------- reboot