Please note not all versions are covered here, but it should give you the general idea.

In Internet Explorer Versions 5.0 & 5.5 go to :

Tools / Internet Options / Security / Custom Level / enable "....IFRAME"

In Opera Ver.6.01 go to :

File / Menu / Preferences / Page style / then enable frames & inline frames

In Opera Ver.4.02 go to :

File / Preferences / Documents / Frames / then enable frames & inline frames