Remember not to select "Automatic partitioning" or similar, since this will wipe the entire hard disk.
Backup your data first!
dd if=/dev/sda conv=sync,noerror bs=64K | gzip -c > /path/to/backup.img(in which /dev/sda is your harddisk(not sda1, that's a partition) and /path/to/backup.img is the location where the image will be stored. Save this image an external drive, if you can). This command creates a so called bitcopy, an exact copy of your hard disk. You can use this to restore your computer, including partitions and mbr(grub/lilo) to the way it was at the time of creating the copy. This is useful to have anyways, whether everything goes right or not.