If you have the latest java version installed can you delete the previous versions

When you're dealing with minor releases (v1.6u5, 1.6u11, etc.) then the answer is yes, you should have no problems removing old versions.

However, if you're dealing with major releases (v1.4, 1.5, etc.) then the answer depends on any installed Java applications you have. Usually an upgrade should not affect any of your apps, but occasionally you may have one which depends on a particular feature (or bug) of an older release in order to work correctly.