If you have 32 bit XP you cannot upgrade to Vista x64. You must do a clean install. You can upgrade from XP 32 bit to Vista 32 bit however.
You can only upgrade 64 bit to 64 bit or 32 bit to 32 bit.
Frankly, a clean install is more desirable anyway (after saving your data).
im currently running windows xp, how ever have an amd athlon 64 process. will i be able to upgrade staright to vista 64? or will i need to do a fresh install. hope someone can help.

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