pc wont boot from cd or HDD

ok so here is my problem, im fixing a pc and never run into this problem. I believe the HDD went bad, although when i ran a bootable cd, for the virus scan.. the virus scan worked, so than obviously the HDD is still readable, but does not boot up.... yes the boot order is correct. So when i go in and put a win7 installation cd in, so i can go to repair and do a system restore.... it boots from the cd but than stops, the console doesnt show up. I put a new HDD in the laptop and it boots from that.... im so confused. any ideas?

it makes me wounder if the MBR got messed up somehow, but how do i fix that if i cant get into the windows 7 bootable cd, repair console?

yes near philly, limrick pa, about 25 minutes away.....

it cant be the disk drive, since the av booted up (3 diffrent ones)

i tried several OS disks, (win7 pro, win7 home, and vista) all same thing, it loads the files, and gets to the graphical screen but does not load the console (with the options like cmd prompt, system restore etc....) it does though give the option to install...mabye ill use the pro cd, and see if i can upgrade it to pro... im scratching my head, this is so wierd...

this leads me to a question, from my understanding, when booting from a disk. that the pc runs of the Ram, and nothing to do with the hard drive, although when i had the other hard drive put in there, the bootable disk booted up fine, which is causing me to scratch my head more lol

hmm thats very interesting theory, ill give that ago after them mem86 test finishes (so far its passing)

thanks guys for all the comments so far

ok booting up, to the bottable win7 disk was much faster, and it went right to the console i wanted..... so this means the hard drive is bad correct? not just the MBR like i orginally assumed?