My computer will not stop trying to install Windows XP

[craigpainter]

I tried to install windows XP on my Sony Vaio but when I got to the part where I needed to put in my product code, I realized I no longer had the package with the codes. At that point I decided to try and cancel the installation. I can not get out of the Windows XP installation. I have turned off the computer thinking that it would reboot normally but it goes directly to the Windows XP installation. I am stuck in a cycle and do not know how to get out.
How can I stop the installation process?

[JJ 3000]

You don't have a product key sticker on your computer?

[craigpainter]

I purchased XP a couple of years ago and apparently lost the box. I am most interested in stopping the installation process since I know that I no longer have the product codes

[JJ 3000]

The XP install asks you for the key AFTER it lets you format (erase) the hard drive.

You will need to buy a new copy of XP, or use a free operating system like Linux.

[craigpainter]

I have Windows Vista that I could install if I could figure out how to stop my computer from trying to install Windows XP. I want to be able to stop the Windows XP installation and install Vista. Any ideas on how I can do this?

[patio]

Boot to the Vista CD...and install.