Mac Mini Boots to Unix command line

Question

I've used my Mac Mini for almost two years with no problem. Normally I can figure out what I've done wrong...but not this time. I need some help from the OS X community.
This morning when I toggled to my Mac Mini from Windows (via a KVM switch - not dual booting) I had my OS X login screen with my username and one other username (Guest, I think). It wouldn't accept my password for my account, and I've never had a "Guest" (or whatever the other name was). In addition three other user accounts were missing (my wife, daughter, and one other). All I knew to do was restart (by turning off the Mac and turning it back on because it wouldn't accept my password).
When it booted up I had a black screen with the following two lines:
-sh: /etc/profile: is a directory
-sh-2.05b#
The only unusual thing I can think of is that last night I downloaded and installed a 'free' version of DivX (from their web site). I've never had a problem like this before.
Suggestions? I'm already having withdrawal symptoms from my Mac. Thanks in advance...
Mac Mini   Mac OS X (10.4.8)   1.42GHz/1GB/80G HD/SD/Bluetooth +AP/56K MDM

Answer 1

Then, I suspect that the profile file is corrupt. Boot with the install disk, select your language, and, when the menu bar pops up, select Utilities->Terminal. When the Terminal window pops up, type in the following command, and hit the return key:
cat /Volumes/"name of HD"/etc/profile
The result should look like this:
# System-wide .profile for sh(1)
PATH="/bin:/sbin:/usr/bin:/usr/sbin"
export PATH
if [ "${BASH-no}" != "no" ]; then
[ -r /etc/bashrc ] && . /etc/bashrc
fi
If it isn't, then my only suggestion is to reinstall the OS, using the Archive & Install option, saving user and network settings.
G4 450 MP Gigabit   Mac OS X (10.4.9)