When I try to start EAC or to compress/decompress, I immedately get an Access Violation. Why does this happen?

Usually this error occurs when not using the ASPI interface of Adaptec. So try at first to install it on your computer. When you use Windows NT/2000, the native SCSI interface could also make some problems, so also install the Adaptec interface in that case.
Second, it has also happened that an illegal audio codec let EAC crash on enumerating all audio codecs. In that case try to disable (not deinstall) each codec and try again, you could enable the codecs then again piece for piece to find out which one causes the problem. Mainly some versions of LAME and OggVorbis ACM codecs cause this behaviour (included in e.g. Nimo Codec Pack).