If you boot from an Ubuntu install CD/DVD or USB flash drive, run the installer, and select the option to install alongside existing operating systems, the installer will attempt to automatically shrink down your Windows Vista partition and create the partitions Ubuntu needs in the newly freed unpartitioned space.
This usually succeeds, but sometimes doesn't. If it's unable to shrink the Windows Vista partition because of files that cannot be moved, then you should try shrinking the Windows Vista partition in Disk Management in Windows Vista. (This functionality is not available prior to Windows Vista; for example, in Windows XP you cannot use Disk Management to do this.) You can run Disk Management in Windows Vista or Windows 7 by typing in
diskmgmt.msc in the textbox at the bottom of the Start Menu, then pressing Enter.
If the Ubuntu installer cannot shrink the Windows Vista partition and you cannot shrink it with Disk Management in Windows Vista, try disabling swap (i.e. paging) and hibernation and trying again in Disk Management. (Afterwards, you can re-enable swap and hibernation.)
For greater customization during installation, you can select Do something else and manually partition (but only if you know how). Alternatively you can manually partition using the GParted Partition Editor on the live CD/DVD/USB before running the installer. But most users who want to run Ubuntu and another OS on the same machine are pretty well served by automatically installing Ubuntu alongside their other operating system.