Tell me are you trying on Oracle NT server/work station. Server allows you to have multiple instances. I don't think, workstation allows. still you can give a try. becreful to follow this procedure.

edit registry with 'regedit' go to /HKEY_LOCAL_MACHINE/ORACLE/DEFAULT_SID and change the default sid you want use. so your database will be dafaulted to this sid when system starts/boots and try to bring your database up explicitely giving the path..

svrmgr>startup pfile=/xxxy/yyy/initxxx.ora