a few thoughts-your user account might be wonky. grab your original install dvd, sign in as administrator and see if you can set up a new user account, fix your access privileges . second-see if you can start up in single user mode, then run disk utility and repair permissions (just a guess)
failing that, try an archive and install. It won't hurt any of your programs and stuff
and it might solve any problems that won't go away....
just my opinion
One more intermediate step you can take before re-install.
Hold down the Shift key at Startup. This invokes Safe Mode. It spends five minutes doing a Disk Utility ( Repair Disk ) and then loads the smallest possible set of Extensions, and presents the login screen, even if you normally auto-login.
If this works just fine, and regular boot does not, the difference is the additional extensions, including those for your graphics card.
If you continue to have problems, you should also run Apple Hardware test from your original DVD, if available.