It won't have any adverse effects on other programs. In Windows Vista, even if you are logged in as an Administrator, normal programs are run with limited permissions. If they need to d things which are considered the prerogative of administrators, they need to request a temporary "upping" of the rights. XP programs can't do this, so will be constantly clashing with UAC. Also, under the Vista guidelines, programs should not write to the "Program Files" folder or the Windows folder, but many XP and older programs do this (maybe because they store their data in a subdirectory of the program folder, or because they save ini files in the program or Windows folder.
By turning off UAC all you have done is remove a level of additional (and questionable) security. It will not affect any other programs (except maybe make them easier to run!).