You will get activation problems since you wasn't able to deactivate on the old harddrive.
You mention C: so I'm guessing your on Windows XP?
In that case, start by installing from your original installation disk/file. Then you could try to restore everything in
i C:\Program Files\Adobe\Adobe After Effects CS3\Support Files\Plug-ins
i C:\Documents and Settings\YOUR_LOGIN_NAME\Application Data\Adobe\After Effects\8.0
- Jonas Hummelstrand