it sounds like you've wiped your passwd file, but it could be a problem with your login manager, at the login screen type [CTRL] + [ALT] + [F1]
if you still can't login there, then boot up your rescue disc (install disc 1, type 'linux rescue' at the boot prompt instead of pressing enter.
thhe type 'ls -lh /etc/passwd*'
if it returns nothing, yo'll have to reuild your passwd file from scratch
if it returns '/etc/passwd' then the passwords are probably wrong
if it returns something similar, like /etc/passwd~ or /etc/passwd-, but no passwd, then do 'cp /etc/passwd<whatever-else-it-was> /etc/passwd'
post what you find anyway
EDIT: by the way, /etc/passwd is the file that linux keeps track of users in