Even if both OS shared a single partition, there would be no major problems experienced - as long as the other Windows had its own directory use a different name, like, say, "WIN64". The boot manager would then provide you the choice of either OS at boot, and that was it. Yes, you had to ensure that 32-bit programs in Win32 "Program Files" directory don't get erased by the same-named 64-bit versions installed by Win64 in that same directory - 32-bit apps under Win64 have their own "Program Files x86" folder, so no problem there.
If you installed each Windows in its own partition, even that program files problem would be non existent.
If you added some kind of Linux, its own boot manager would usually install on top of the Windows one, offtering you a choice of various Linux boot levels and the Windows boot, which could go into the above mentioned Windows boot manager if there were more than one Windows installed. Of course, Linux file system is vastly different and needs its own partitions. Most users deciding to go with Linux know that well, and usually prepare their hard disks accordingly - so no problem there, either. You can have a Linux and a bunch of Windows together this way.
Now, what happens with Vista? I tried to install Vista Ultimate 64-bit edition on my quad-core Xeon system, which already had WinXP 32 and WinXP 64, each in its own partitition. The first try was to install it in the same partition as one of existing Windows. Unlike the older versions, which are more friendly to the notion of co-existing, Vista insists to disable the old Windows and move all its contents to WINDOWS.OLD folder, essentially robbing you of your old (and often, dearly paid) licence, apps, user settings and so on.
When I selected install in a fresh partition, everything went fine then - which, after all, may not be a bad idea, as a new, unproven giant bloat of an OS could still have unknown bad effects on the exising OS (as if that one isn't compromised enough).
So, if you want Vista of any kind to co-exist with your existing Windows on the same PC, you either have to ensure that your HDD has enough space for another partition (or resize your sole Windows partition using tools like Disk Director, which not every user has), or add an extra hard drive. Which is a pity, since many initial Vista 64 users may have to keep the WinXP 32 still installed - if they want to run many games or apps broken by the Vista, not to mention device drivers as well.
Insterestingly, WinXP 64 has far less game compatibility problems than the Vista 64, according to a few user's feedback. So it could more likely afford that 'clean-sweep' upgrade requirement as there is less need to keep the old 32-bit version - if you have all the necessary device drivers for it, that is.
Why couldn't Microsoft do a simple compassionate move and allow this easy coexistence of Vista with older Windows on the same partition? Another element in forcing the uneducated layman end user (like those journos reading press releases with "light bulbs" and "shower heads" from the Intel Penryn story) to unknowingly or ignorantly get relieved of its existing (often pre-installed) Windows, and become solely dependent on the new DRM-infested (and, for the same hardware setup, slower) malware OS? Would be lovely to know... as of now, Vista install over an existing Windows is no better than a, say, fresh Linux setup. So, you may just as well consider the latter one - less, if any, cost involved.
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