How to Boot Up From Disc? (Pressing C while starting up doesn't work)

I was trying to boot up from my Snow Leopard disc last night to run Disk Utility (to repair my hard drive). I pressed C while starting up but it took me right to my login screen. I did this several times, no luck. I'm using a 15" MBP btw.

I also tried the same thing on my sister's MacBook (using her own install disc) -- same thing, straight to login screen.

I know pressing C used to work before because the MacBook used to belong to me and I did an erase & install before passing it down to my sister.

Is there anything I'm doing wrong?

 

Tried that too (both with the MacBook and the MBP) and I got a picture of a lock. I couldn't do anything except hit the power button to restart again.

Could that mean there's something wrong with both hard disks? Ran a disk check on Disk Utility and it says that my hard drive "appears to be ok." (for the MBP)

 

That's it!

I set this up a few months ago and forgot all about it.

Thank you so much!