I sorda have a program, but for Windows logins.  Actually, it's possible to check whether they have entered a correct password or not, so you can make it even less suspicious.  Actually, what I was trying to do is, after they enter the correct password, it launches explorer.exe in their username, so it carries all their settings etc.

My only problem was Ctrl+Alt+Del - no way to take over that with the permissions given...

Hmm, now that I know about rootkits though - I remember seeing something which hooks the process and records all passwords used :P