First, you should verify F12 is really your boot selection key. The boot selection key is different for each computer manufacturer and sometimes is different between specific models. Also, generally you shouldn't just hold down the boot selection key. You simply need to press it at the right time. By holding it down, you may inadvertently be canceling the boot selection selection screen when it appears. Try repeatedly pressing and releasing it as the computer boots (slowly) as opposed to holding it down.
Additionally, what live OS are you trying to boot from? Have you used the live OS on a different computer successfully? Depending on the age of the laptop, there's also no guarantee that the computer supports boot selection. You may have to enter the BIOS setup and simply change the boot order.
Once you've worked out your live CD problem, if you're using Windows you could try using a DaRT recovery disk or Windows RE disk to boot and possibly repair any Windows specific problems.
Something else you should do is check the health of the hard drive. If you're using a Linux live CD, most of them come with smartmontools, so you can simply run
smartctl -t long /dev/sdX where X is the appropriate letter for your disk (generally this will be
a, however I see it as
e frequently when the computer has a media card reader). You could also remove the hard drive and connect it to another computer and use Western Digital Data Lifeguard or Seagate SeaTools for Windows to test it (note that you should probably select the one that matches the manufacturer of your hard drive). Note, you should also examine the S.M.A.R.T. data for reallocated sectors, and read errors.
If the hard drive checks out to be okay, hook it up to another computer or use a live CD to look in the Windows folder for a folder named
Minidumps, inside this you may find several
<date-time-stamp>.dmp files. If you find these, you can use the Windows Debugging Tools, or DaRT to analyze them and possibly determine the cause of the problem. You can also upload them and I or someone else can analyze them for you.
You should also consider doing a memtest on the RAM. If using a Linux live CD, then most of them come with memtest86+.
If all else fails... Take it to a professional.