My 14 year-old son is using a Ctrl+Alt+Del procedure to get into my computer even though it's password-protected. How can I prevent him accessing my PC? He got that procedure on the web.
Go into the BIOS (usually F2 or Del at boot) and set a boot password. This will be required every time it boots up. Every BIOS is different, so I can't give you step by step instructions, but it shouldn't be too hard to find. This will prevent the computer from booting into any operating system at all, until the password is entered. Keep in mind that you'll still have to put your login password in too, after it boots.
From a non-technical angle, as a 14 year old I would've gone nuts without access to a computer. I don't know what your situation is, and far be it for me to give you parenting advice, but consider giving him access!
Assuming Vista or Windows 7, the alternative would be to give him his own login account, with limited privileges. This prevents him from installing anything, and he can't get at any of your files within your user profile (e.g. My Documents, Pictures, etc). You can do the same on Windows XP, but the limited user design isn't as robust. This would allow him to browse the net and whatever else, without him interfering with any of your documents.