If I understand correctly:
Under no circumstance should you install Vista on a computer you actually
intend to use.
If you upgrade Windows XP to Windows x86 Vista Beta 2, you will not lose
your document files. It is still absolutely a must that you back these files
up to another hard-drive or burn them to dvd's or something before ou even
think about upgrading. There is a significant chance of your existing OS
being compromised during the Vista installation which would result in the
loss of your files. Once you upgrade, there is no way to uninstall Vista; to
go back to XP you must re-install XP and then you may lose everything.
That's why the back-up of your files.
A better option is to add a new hard-drive or make a partition on your
hard-drive and install Vista there instead of upgrading. If you want
instructions for this, search the forum since it has been discussed 82
different places. Then you will have the ability to "dual-boot" XP and
Vista. (There has been talk of duel-boot systems, but swords ave always
scared me!)
Windows x64 Beta 2 can only be installed by reformatting, either your entire
hard-drive or into a new hard-drive or empty partition.
Hope this helps.