Could probably work, but is there a possibility to install over the already existing OS? I don't want to do it again from scratch, it's fully set up now and it took me long time.
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Sadly not. You can upgrade an existing version of Vista to another (Premium to Ultimate for example), but you cannot upgrade a version from 32bit to 64bit because the codebase and actual OS structure is quite different.

When you actually use a 64bit version of Vista you instantly notice things are a little different. The Program Files directory is split into 2 (one for 32bit and one for 64bit applications) and the same is true of the registry which is split into extra branches to cater for both code bases. And other things like the Control Panel have native 64bit versions installed.
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