Multiple Selection (Vista and Win7)

Hi, I select more than 15 files in Windows Explorer on Vista and right click, my
options are reduced. For example, if I select 15 .txt files and right
click, on the menu I have "Open". But these options are
gone if I select 16 or more files. Similarly for other files. e.g. If I
select more than 15 MP3 files I can open them only in Windows Media Player
and not my default player. Can I fix this?

Thanks.