John, what brand name of computer do you have or are thinking of buying? The procedure varies with the brand. But it's simple to bypass the lock.
Here's a sample procedure, so you can see what I mean. Pretend your computer is a Dell. ANY Dell.
1. Start with machine off. You press the poweron button and immediately start hitting F12. Do it every other second (not too fast) until you see in the upper right corner, 'One-Time Boot Menu'; should be in yellow, but might be white.
2. You're next presented with boot options. Select 'BIOS Setup' with cursor keys (or mouse, maybe).
3. Now you're in the Bios Setup menu. Slowly scroll down and look for something like 'Boot sequence'. This term means, 'Hi, I want you to look in x place FIRST SECOND THIRD for the boot instructions'. So, you want the CD and/or USB port to be first and second (or floppy, if you use floppy), hard disk after that. Thus you can always boot with any other device before your hard drive. Handy, if you want to test drive different flavors of Linux, prior to install.
Adds maybe 30 seconds to your boot time, but so what? This means your computer first looks to your CD and/or USB or floppy, before looking on your hard drive, for the 'boot' instructions.
4. Still in the 'Boot sequence', look just below the ranking of what-boots-first, and you'll find two labels: 'Legacy', or 'UEFI'. You want 'Legacy'. (It's 'UEFI' which Windows 8+ uses to prevent another OS from being installed. I oversimplify, but that is what you need to turn off.) So make sure 'Legacy' is selected.
5. Now this is the tricky part: in Dell, you scroll almost all the way down to 'POST Behavior' (meaning, how your machine checks itself at boot): look for 'Fastboot'. You'll have three options for its setting, Minimal, Thorough, and Auto. Pick 'Thorough'. Technically means, machine does longer diagnostics of its drive, memory, monitor, etc. In reality it stops Windows from interfering with the somewhat longer boot process of Linux, so Linux can control the boot process. For Linux, displays information (usually, from what I've seen); maybe you want to read it. Minimal and Auto are a lot faster, but maybe only use those after you're sure your dual setup is stable.
Problem is, each brand has its own names for these things, though the terms should be similar. Also, some brands use different start keys. For Dell, it's F12, the most key among brands; Acer is sometimes Alt-F10 or F10, some are F2, etc. I forget what HP is.
One more small thing: partition and format your drives before installing Win10, so it won't force you to have GPT formatting. Use NTFS, partition 160 GB or so (it's a real swap hog, makes huge files for its many updates). It will subdivide the partition you give it into a 450MB for the mbr and the balance for its operation. Either leave the Linux portion remaining, as unallocated or format also as NTFS, for now. The Linux installer will guide you into reformatting what remains (every distro has its own idiosyncracies in that regard). It will create the grub loader, too.
I did all the above steps (except for installing Linux) plus the paritioning before installing Win8.0, then upgraded to 8.1 via update, then installed Win10 in its own pre-created partition. Both were NTFS, not GPT. The Windows installers will create GPT if you let them; but they don't play well with Linux partitions. Windows will not be able to read Linux partitions, either. So always install Linux last. So far as I know, all the Linux folks here would agree with that, but ask them. I only report what actually happens to me (or I've read/heard), can't speak for all experiences.
TMI? Lemme know!
Edited by brainout, 17 July 2015 - 12:25 PM.