LR 6 needs you to run a 64Bit version of Windows 7 or later.
Since this forum is a user-to-user forum to help out on Lightroom issues, you might need to go to some other place like Microsoft forums to get help in upgrading your OS - Sorry.
You can upgrade from Vista to 8.1:
Upgrade to Windows 8.1 from Windows Vista or Windows XP - Windows Help
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