Grub messed up on dual boot machine - can't boot live usb

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http://askubuntu.com – I have a laptop on which I had Win 7 and Ubuntu 12.04 installed. The hdd has two partitions (one primary and one extended). Grub wasn't loaing initially so I ran the boot disk repair tool which seemed to sort it out. However, I mistakenly removed python (long story) from the Ubuntu OS and this obviously messed up Ubuntu a lot. So I decided to reintsall both Win7 and Ubuntu 12.04 in an attempt to get things back fresh. Win 7 went fine, but now when I try and use the Live USB to install Ubuntu I get: error: no such device: grub rescue > I guess grub is still there despite the Win 7 reinstall (HowTos)