MacBook Pro won't boot into safe mode or from DVD Specs: MacBook Pro 7,1, 2.4gHz Intel Core 2 Duo, 4GB DDR3 RAM, 500GB HDD with Snow Leopard v10.6.7 

I'm having a weird problem. I want to boot from the install disk to run Disk Utility because I'm having a Time Machine problem (that's not the problem). So I grab the install disk and hold down C as the computer starts up. The logo appears and then it goes straight to the login screen. I did it again, still the same thing.

So I double check and saw that Startup disk is set to Snow Leopard disk and there is nothing plugging into the computer besides the power cable. 

So I did it again, still went straight to the login screen. So I tried booting into safe mode by holding down shift key, still the same result. Also tried it with booting into the Hardware Test with the Application Disk, still booting straight into the login screen.

The computer itself works fine, in fact I'm typing with it. The DVD drive works because I just finished playing one of the West Wing DVD and the supplied Apple disks works and load setup just fine.

There have been no hardware modifications in the last 6 months.

Can anyone help with my weird problem please?
Press and Hold C button should take place immediately after you switch the MBP on. Delay could cause it to go with the normal boot sequence. Continue to hold until you see the gray screen with the apple on it