เหตุผลที่ยังไม่ควรใช้ Windows 64 bit จาก http://www.tech-recipes.com/microsoft_vista_tips1426.html
1. Most hardware does not currently have 64-bit drivers.
Out of all the boxes that I have installed x64 Vista on, I could always get it to boot up. However, the lack of 64-bit drivers for many hardware devices typically left me without any chance of burning DVDs or listening to audio. Networking devices and card readers were frequently not supported as well. Vista x64 is pretty but it's not very fun without network access or audio.
2. In Vista x64, any driver that is not properly signed will not be able to enter the kernel and will fail to load.
Think how many times you have ignored that warning that a certain hardware driver is not properly signed. With vista x64, if your driver has not be blessed by Microsoft, it will not work. Forget about it.
I have tired to get around this by booting with the F8 option Disable Driver Signature. It doesn't make a difference.
3. Vista x64 currently does not backward support most x86 (32-bit) drivers.
For the most part 64-bit systems run 32-bit applications very well. However, vista x64 doesn't run x86 drivers... at least at this stage.
4. Vista x64 does not support 16-bit software.
You may think that you never, ever run 16-bit software. However, XP actually handles this legacy fairly well.
5. Very little x64 software currently exists.
x64 software runs better on a x64 system with an x64 OS. Currently, however, there is very little x64 software out there. If you have that magical combination, you do get a nice performance boost. Currently, however, this combination is way to far ahead of the curve.