See the note on the consumer-facing Java website regarding 64-bit vs 32-bit versions of the JRE:

> 64-bit Windows operating systems (which may be Windows 7, Vista or XP) come with a 32-bit Internet Explorer (IE) browser as the standard (default) for viewing web pages. These operating systems also include a 64-bit Internet Explorer browser, however using it is optional and it must be explicitly selected to view web pages. Note that because some web content may not work properly in a 64-bit browser, we recommend using the default 32-bit browser and downloading 32-bit Java.

More details: http://www.java.com/en/download/faq/java_win64bit.xml