Thank you, it's a laptop- Dell PC, with windows vista and Intel, and I wanted to give it away but I don't want my info on it.
That's the commonest reason for your question, but I needed to be sure.
I'll assume for present purposes that you only have Drive C: on this machine. A Dell laptop which is sufficiently recent to have Vista will have some arrangement for restoring the machine to factory condition. Most likely it is a recovery partition with one or both of (1) a program to make a restore disk or disks (from which you will boot the computer to run them), and (2) a program actually on the machine that does a full factory reset. You need to run one of these; (2) is usually easier. Your Dell documentation, which is as likely to be in the form of pdf documents on the machine as of a paper manual, will have the details of how to do this.
The factory reset will format your C: drive, and install Windows in the state in which it originally came with the machine. Your personal data will not be visible, but it will still be recoverable, at least in part. You need to install Eraser on the newly reset machine, and run a free space erase on the C: drive. This will 'complete with errors'; when you look at the log, you will find literally thousands of entries telling you that cluster tips could not be erased on particular files. Do not worry about these errors; if, at this point, the erase has otherwise completed normally, your personal data will not be recoverable. More details about the process are in the FAQ topic I referred you to in my previous post.