If you have to operating systems, at least this is the way ot works for me, it automatically asks you at startup.
Originally Posted by spokenfor
Right-click on "my computer" and select "manage." Then, under the "disk management" screen, right click on the physical drive you want to make a new partition on and "shrink volume" to the size of the new partition. Right click the new "unallocated" partition and click "new volume" and fill the the volume label and select a drive letter.
However, windows disk management is not always able to work around fragmented files, so if it's a drive that has been used for a while and isn't well defragmented, you might not be able to "shrink volume" to a large enough partition size for a new OS.
Best of luck.
Also, it will ask you which OS to boot up when you startup the computer.