public osx login (getting anywhere)


i initially installed osx before 9.04 and have severe problems. (wrong way to do it i know)
after re-installing osx on top of 9.04 im unable to get any further than the login window
if i fill in the orginal username/password it just shakes as if they were wrong data
im not even able to start up from any system cd (tried 9.04, osX and even 8.1) using the c-button on restart (i also tried holding down alt-command-shift-del)

the only way i can get anything else than this login window is by resseting pram (command-alt-p-r) which will get me to the shell window. this is very fine apart from the fact that i dont know what to with it (i know basic unix commands like cd, rmdir, rm, ls and so on)
i have tried to remove some files but this proved to be impossible via the shell

so my question is:
a) are they any way to login if i accidently forgot my original password?
b) is it possible to uninstall all of OSX when im not able to login in the first place
c) is it possible to uninstall all or some of OSX (the username file for example) via the shell

really hope anybody can help as i cant get anywhere