I think you are talking about creating a dual boot setup where you have two (or more) operating systems installed on the same partition. Here is a tutorial
on how to create a dual boot machine. With creating a dual boot computer there is always a risk of corrupting the boot loader rendering both operating systems inoperable.
If you do not care for Vista and don't plan on using it, look for a computer with XP - yes I know they are hard to find. Dell Business Line will downgrade
the computer to XP for $99.00. Since you are shopping for a new computer, see if you can find one that offers a free upgrade to Windows 7.