I know you can choose the archive and install option when installing Mac OS X over an older installation. But I have to admit that I don't know if you can do the same without deleting your non-OS data that's already on the disk.
Anyone?
Not to make fun of you – I feel your pain – but your situation is exactly why I make regular clone backups of my MacBook's internal hard drive on both a 2,5" external drive and a 3,5" external drive. Especially the former is relevant here, as that allows me to be back up and running by simply replacing my MacBook's drive with the drive inside the 2,5" enclosure in case of drive failure.