Since you only want disadvantages, I'll shed light only on that:

* First disadvantage is that even now not many programs come in 64-bit. A good example is that there is no 64-bit media player. So eventhough it you can still use 32-bit players, you are essentially wasting the additional processing power by not using it. PS doesn't have 64-bit version either.

* 64-bit machines draw more power and heat up more due to the same.

* 64 bit drivers are still a big issue.

* Many devices are incompatible with 64-bit OS (apparently even after installing drivers, though this is rare.)

* Some language compilers are reported to have issues in a 64-bit environment.

* Not many games use 64-bit capabilities.

* although most 64-bit OS support 32-bit applications, some 32-bit applications do not support 64-bit OS.

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These are some thta I remember from top of the head... google for a far indepth knowledge.

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But do not let that put you off. The simple matter is that since 64-bit is becoming more popular, many application exclusively for 64-bit have been written. Most worthwhile 32-bit applications will work on 64 bit. If they do not, it simply means that they have a 64-bit version.

The advantages of 64-bit outweigh its disadvantage. In case of Vista, the biggest and the most prominent advantage is that only the 64-Bit Vista supports more than 4GB RAM.