iMac can't start

Hi,
Need help, have problem with my iMac,
My iMac can't start, when 'on' the start button, it appeared white screen for a while and apple logo came out followed by logo that looks like no entry sign. These two logos toggle between each other forever. What could be the causes Thanks for any help.

iMac spec:
24' 2.8 core 2 Duo
500 TB HDD
3 GB Ram 667 DDR2
OS Snow leopard

What were you doing prior to this? Are you trying to install an OS? A little more information about what you have or are in the middle of doing might help.

This type of issue can be seen when trying to upgrade/install Mac OS while using the wrong type of install media.

Anyway, you need to provide some further info

Hi Marty,
Thanks for your response,

2 days before I installed 1 pcs. of 2gb 667 ram.
A day before I installed a 500 gb Seagate Free Agent Go Flex external hard drive.
I set to Paragon NTFS to enable the drive to be use on Mac & PC. It works fine after the installation, but problem occured the next time I ON the machine.
I explored the the setup folder but couldn't remember what I had done to the setup. Is the problem serious ? Any way to get back to OS ?
Thanks for your help.

p.s. Typo error, internal hard disk is 500 GB. not TB

So ... you have recently installed Mac OS X ? Can you describe the disc/s labels? You have installed an internal HD or external HD?

Hi,
I didn't installed or upgrade Mac OS X.
Recently I installed Seagate 500gb external hard drive (usb 2.0 plug & play)
Could it be the external hard drive caused the fault ?
Thanks.