Hopefully this will answer your questions:
1. Check your PICKADD variable.
2. AutoCAD 2004 files cannot be opened by any previous version. If you need to open them in 2002, use the "save as 2000" option in 2004.
3. 2004 is not supported on Windows 98 -- if you're trying to load 2000, though, I'm not sure if that would cause problems. I bet somebody else out here knows, though. :-)
Hope this helps -- feel free to come back with more questions!