When you install Win7, it will show a screen when you boot up to select an OS. I have XP, Vista 64, and Win 7 64.

If you install XP after having Vista installed, it usually screws up the booting and will only boot up XP. You will need to put your Vista disc back in and have it repair boot options...easy to do, it gives you that option and you'll be all set.

yeah, and you can always install a boot mananger/loader.....

http://gag.sourceforge.net/pics.html

The first sector on a hard is called the Master Boot Record (MBR). This sector is only 512 bytes long and contains a small piece of code (446 bytes) called the primary boot loader and the partition table (64 bytes) describing the primary and extended partitions.

By default, MBR code looks for the partition marked as active and once such a partition is found, it loads its boot sector into memory and passes control to it.

Och lite mer intressant läsning om boot

http://msdn.microsoft.com/en-us/library/aa468626.aspx

I personally would never recommend using third-party bootmanagers, having had some bad experiences with such in the past.
One caused me not to be able to enter any of the OS's installed. Another nuked the MBR.....

Safe to says that I haven't used any third-party managers since