This is probably going to be post #6, and you still haven't told us whether this computer is a desktop or a laptop, never mind the make and model !
If this is a desktop why not just install this hard drive as a second drive and use it as storage ? You could move much of your data across to it to free up space on the drive with your OS.
If it is a laptop with space for only one hard drive then you are going to have to find some way of changing the boot priority. Have you tried 'Delete' ? that's a very common BIOS access key. But even this will not give you more space if you intend having the same applications and data on the drive.
Other things you could try to free up space is run Ccleaner - a free download from BC. It's a very useful little utility for removing temporary files. One word of warning - Ccleaner includes a registry cleaner. You are recommended NOT to use this function.