I have an iMac with OS X 10.4 that I can't login to. Whenever I get to the login page I type my password and it starts to login. The blue and white status bar appears and eventually login but only for a split second. A blank blue screen flashes and then it returns to the login screen. This only occurs with the admin account. Any ideas on how to stop this?
Try rebooting in Safe mode by holding Shift as the Mac starts to boot -- this will disable several categories of system add-ons (see Apple's KB #HT1455 and #1564) that're likely suspects with a problem of this sort. If that works (i.e. lets you log in), look through the various things that were disabled, and experiment with removing them some at a time to isolate what's causing the problem. Since it's only one account having trouble, things inside the user Library are the most likely suspects.