Why did my dual boot kill a computer

I recently lent a hard drive to a friend, on which we proptly installed linux (so his computer was dual booted), later i had to take back the hard drive and (to make a long story short) we ended up having to completely re-install windows. can anyone tell me why this happened and how to prevent it in the future, by the way the linux was SUSE 9.2.

on what drive did you install which OS?

First, what exactly happened? Your system started smoking? Big green flashing rings started to float around the screen? You could only boot into Tetris? The computer made funny "bleep" "bleep" noises? Really, how can we explain something you don't describe?

Operating systems can not boot if the hard drive location changes. For example, if the drive was slave on your friend's system and master in your system, then the bootloader got confused and probably either crashed or gave you some weird command prompt.