To Reformat your HardDrive, which general means you have a clean system to start everything from scretch you 'google' it with the terms:
'how do I reformat my hard drive'
To create a 'Bootdisk' go here: Boot Disk
You do need an 'Operating System' CD to install it back on your computer. You get it with every new computer, you could use a friends, buy a new one or get a copy.
Your friend will lose any important data but he could save what is important to a floppy or CD.
You should 'google' for problems that arise from reformatting, it is not the 'be all and end all' but it is the answer to most pressing problems.