how do i do a ŕepair to core2

[david]

hi people
i do a fair bit of upgrading and tend to change mainboard and cpu and
sometimes vga but keep the hdd as is with all my data on it.
usually it becomes unbootable or to say the least is very hard to see
i run the install disk and try to upgrade and it doesnt reload the new
hardware or if ive lost grub replace it.
question: is there a command which will force a repair type operation
similar to the repair in xp where you hold all your files and just the
system, and hopefully the new hardware is found and loaded?
thanks
david