1. Why dual boot
Simply because they have more than 1 OS (operating system). You
can't boot up to 2 OS at one go, like it or not.
2. How to dual boot
It's not that hard if you know how to install an OS.
If you want to dual boot the same OS (Windows 2000, XP and Vista),
install another copy on another partition.
If it's different versions of Windows, oldest version of Windows to
newest version of Windows is to be installed in that order.
If it's Linux and Windows... Windows must be installed first,
followed by Linux. During partitioning in Linux, don't delete and
format the Windows partition.
Usually for Linux, you need to choose advanced partitioning so that
it will be done correctly. So far, the only Linux I know of that
will detect Windows and do the partition accordingly is
OpenSuse.
If it's Mac... not so sure. Never had a Mac.
