Imac wont boot

Hi, I have an Imac with Leopard installed and after an attempted restart, all I get is the blue screen. It never progresses.
Ran disk utility, it said the hard disk was fine, but it still wont start.
Ran safe mode, can get to the password, putin the correct password but it then goes blue and comes back asking for the password again.

Any ideas

Try booting with your Leopard DVD and run the Apple Hardware Test. See the following Apple KB article: Link Be sure to run the extended tests.

Also try resetting the PRAM and NVRAM. Link

Regards.