In this month's Personal Computer World (UK edition - red cover) there's a really useful feature on transferring all your essential files from one PC to another. I reckon that you could also use the guidance given in the article to transfer settings and files between new HDDs if you want a pretty clean re-install.
I agree with other comments and think that the best way forward seems to be a re-install of most applications as well as the OS.
It takes a while, but it's generally worth it.
Have fun
