Here is what I would do, but you do this at your own risk which, if you understand what I am suggesting, is really no risk at all.
First make sure that all xampp components are stopped (not running) and you have closed the xampp control panel.
Also you should be logged in to Vista as an Administrator or a Super Administrator
with UAC disabled.
Drag your xampp folder holding down the Ctrl key so you create a folder called Copy of xampp
in the drive it is already in for example the C:\
drive on a default installation.
You will now have 2 content identical xampp folders (a backup) - C:\Copy of xampp
Now follow the instructions in this recent post PHP 5.3 RESOLVED!
which seems to have worked for the OP, but I have not tested this method to date, so your feedback would also be useful and helpful for others reading your topic.
Perhaps the version you may require would be the zip of this:http://windows.php.net/snapshots/#php-6.0-ts-VC6-x86
Or the zip of this:http://windows.php.net/snapshots/#php-6.0-ts-VC9-x86
The VC6 and VC9 differences are explained in the left column of the snapshot page...
(...but using this backup method it matters not, as if one version fails then you simply try another version).
Perform the above procedure in your C:\xampp
If you are successful and confident that the procedure is working as intended, then simply delete the C:\Copy of xampp
folder and you are up and running with php6.
Now the no risk part is this, if you have problems with getting the above instructions to work for php6 in the C:\xampp
folder, then you simply delete the C:\xampp
folder then rename the C:\Copy of xampp
back to C:\xampp
and you will be back to your original xampp installation with absolutely no problems at all - just start your xampp components as you would normally do and you will see that your original installation is indeed intact and working just as it did before you made the php6 attempt.
Of course the alternative is to wait until php6 is included in a later version of xampp.
Please let us know back if you had success or not with installing php6 and post in the above resolved topic if you want the OP's help with his method.
BTW you can use this method of creating a Copy of xampp
folder (as a backup copy of your xampp installation) when ever you perform any sort of risky procedure that you may feel would create a disaster if those changes went a bit wobbly or even went belly up.
Procedures that come to mind are major changes and uncertain edits to configuration files and databases in your added web sites and of course in xampp itself.
The often difficult and unpredictable procedure of upgrading xampp to the latest version or even going back a version or two for experimenting with the included xampp component versions would also be an advantageous reason for creating this kind of very easy backup method.
The reasons to make this kind of backup have no limits and I use this method all the time without a single issue when ever I have had to revert to my original installation from the Copy of xampp
Good luck and my very best wishes.