Check your library path. I've had this sort of problem before (I think when dual-booting between Windows 98 and 2000, with it only installed on one OS).

Tools->Environment Options->Library->Library Path

If it's blank then you'll get all sorts of crazy errors.

You can either manually add in the important Delphi dirs (bin, lib, and so on) until the components are found or you can manually add in all the standard packages IIRC.