It's impossible to know what exactly got changed, but moving the disk to another hardware platform and booting it is a very bad idea, as you have already figured out. 
I'd opt for the clean install, and then invest in something along the lines of Norton GHOST to make image backups of your boot partition. GHOST has saved my butt serveral times when I've made big upgrades and didn't get the result I expected.
I'd opt for the clean install, and then invest in something along the lines of Norton GHOST to make image backups of your boot partition. GHOST has saved my butt serveral times when I've made big upgrades and didn't get the result I expected.
