Trying to install new O/S with dual-boot is a scary job, it can leave you with a messed up bootsector.
To backup first 512 bytes:
dd if=/dev/hda of=bootsector.img bs=512 count=1
First 512 bytes on the hdd is known as bootsector.You should change the device to reflect your boot drive (it may be sda for SCSI).
To restore use:
dd if=bootsector.img of=/dev/hda