I don’t think I needed to get rid of old drivers, so long as they’re no longer being used.
If the drivers are still loading then they are using memory and processor cycles, and they might conflict with other software. Just because you load one driver does not mean that you have unloaded another driver (although that can happen.)
If you are interested, Mark Russinovich's autoruns utility will tell you what is loading, and enable you to decide what gets loaded:
http://technet.microsoft.com/en-us/sysinternals/bb963902.aspx
But, that could lead to a lot of work. if the computer works and seems stable you may just want to leave it alone. Sounds like you got lucky.
Oddly enough, I tried your technique by mistake earlier this week, when I booted a drive on a new Asus mobo that had been created from an image of a C drive with XP Pro that came from a computer with an older Asus mobo. The computer went into a reboot cycle before I figured out that I had connected the wrong drive.