|To delete the boot file|
Identify the OS. If xp then unhide and look for ntldr.exe, ntdetect.com, bootsect.com [if it exists], boot.ini
If 9x or similar its msdos.sys, io.sys, command.com, autoexec.bat, config.sys.
You have to change your view in explorer to view hidden and system files or you won't see them.
I don't know of an easy cheap way of removing the active bit for the primary partiton. You can change this and not delete any files and the disk will not boot.
Easiest is using a program like partition magic or similar.
Otherwise Google for the latest version of fdisk and put it on a boot floppycd, boot it, run fdisk, go to the drive and remove the A active bit.
You will need to go into the bios first and make this the only drive. Fdisk was primitive which is why it requires some extra steps for it to see everything correctly. This also protects the primary disk from an accidental mistake.
You really don't need to format the drive. But if you do, do it thru disk management. Make it an extended partition for the entire drive. It will never boot in any system after that but will still act as a normal storage device.