You are saying that you cannot log on to your desktop and pass the log-on screen? If you wish to "start from scratch" you'll have to reformat your hard drive and do a reinstall. You can also use (if available) password removal software in which you insert a CD, set your bios to boot from optical drive first, and before booting into windows it will give you the option to delete any current password.
If you do not have the ability to carry out any of the above-mentioned tasks, consult a techie who does.