I had a fucked up installation where only GRUB2 survived. The machine CD drive is broken and the BIOS is so old that it doesn't allow booting from USB.
1. Install an ISO onto an USB drive using pendrivelinux.com.
2. Insert the USB into the fucked linux and switch it on.
3. On the grub prompt, type
ls (hd
and press the tab key to auto-expand until you find the USB drive, it's most probably
ls (hd1,msdos1)
4. Type
set root=(hd1,msdos1)
5. Type
chainloader +1
6. Type
boot