Lost Admin Logon and Password

So I am a Windows guy and I took over internal IT support for a small company.  The previous administrator became very ill and is now gone.  Unfortunately no one knows the logon user name and password.  I've tried hundreds of ideas based on what I knew about the previous admin with no luck.  This is a Linux Red Hat Enterprise 5.0 system.  I really don't want to wipe and reload until I figure out what is going on with this server.  Any ideas??  I am not a Linux guy but I learn very fast.