Okay, so you got a laptop or PC from somewhere, cheap, or even free.
It has the entry-level MSWindows OS on it.
It has a moderately large hard disk, say 320 MB. But when you look at the hard disk's properties, it says that there is a nice chunk of it that seems not to be in use. 16 MB or even 100 MB.
And you think, "Huh? I want more space for my movies and songs!" And you format that space. And suddenly part of your laptop is no longer working. Maybe you can't even boot it at all.
You are not alone.
To understand the solution, you need to understand what happened.
There was probably another operating system in there, and it was hiding in the partition you thought would be great to use for your movies. Not just the other operating system, but the program that lets you choose which one to boot.
So, no way to choose. No way to boot. Now what?
If you have a BIOS that is not locked down, you can boot a live CD from one of the Linux distributions. Knoppix is pretty good for this kind of thing.
You'll probably need a real-world friend to help you get the live CD and then to help you walk through the steps.
Oh, you'll need a CD or DVD drive, too.
Boot the live CD.
Launch a disk utility program. This is scary, because the one you want is also able to wipe your whole hard disk clean.
Select the hard disk.
Set the boot flag to the partition that contains the MSWindows boot programs. It will likely be the very first partition, unless the first partition was the one that was supposed to have Linux in it.
Quit the utility program.
You say, that's scary.
Okay, then here is the better solution:
Just install the OS from the live CD. Tell it to use the partition that you erased. The installer will most likely find your MSWindows partitions and set the new boot manager up for you.
And you'll be back in business with a reboot.
I mean, seriously, the only other alternative was to buy an official, high-priced MSWIndows package and try to install that. Just as dangerous and more expensive.
Just don't erase that Linux partition again. In fact, when you have time, boot the Linux OS and see if you can see anything that makes sense. A little bit at a time, and you can be amazed at what Microsoft won't let you do.