Urm - I think we are still missing some info here. If it tells you the time of your last login, and if it is trying to execute commands, then you must have logged in! Are you sure you are not being prompted to _set_ a password?
Try typing 'whoami' to see what that says. Did you set a root password? Try logging in as root. Are you sure the installation completed fully?
Also the startx command may not be in your path by default, so you may have to 'locate startx'. It's probably lurking in /usr/X11R6/bin or somewhere. However, if Mandrake couldn't get X configured on the first go you are in for a bumpy ride doing it manually. Maybe try the 'VESA' driver, or try booting a Knoppix CD and see if that can generate a working config. You'll need to look for an XFree howto, and do some head-scratching.
Keep at it!