Easiest way is, to add a second physical HDD, but with suitable tools it's also possible, to shrink an existing partition, to make space available for an extra one (at least).
Regarding different Windows versions, you should always install the oldest one first.
Even at install time, when operating from the install CD and installing to a big enough HDD, you can already set up more than one partition, or at least give the actual one just a fraction, to reserve free space for later use.
After having installed the first one, you can use the drive manager to add more partitions, resp. an extended partition and logical drives in it, on the unused disk space.
A second Windows version installation will then recognize an older existing one automatically and ask you, if to update or use another place for itself.
If you select an other place, it will install a boot manager with the first one and set itself as default there. But this can later on be changed by editing the boot.ini there.
Same goes for a third version later on, it will add itself to the boot.ini, update some boot files and then you edit to your likings...
However, this is about Windows basics, and therefore it's far off topic here. Perhaps get a good book on Windows essentials.
Linux systems instead would - by default - use the Master Boot Record of the (primary) hard disk to install a universal boot manager.
There are even more chances to prepare fo multi OS boot, like using a Knoppix Linux Live CD for partitioning. Or, if you still have MS-DOS at hand, use XFDISK and it's own MBR boot manager then.
Or start reading here:
http://swissknife.sourceforge.net/docs/
and look there:
http://sourceforge.net/projects/swissknife
There's tons of choices for partitioning and similar, but in any case, only one should be used on a system.
Some can as well shrink living partitions without data loss.
May the power be with you
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Regarding the technical issues on Vista and BDA:
@ Prog
please continue here
I don't have Vista here to play around with, nor would I even want to...
