Perhaps you mean the 'alt' attribute for image tags? If that's the case: You should not use 'alt' attributes as some sort of title/description/etc, it is a fault in IE that pops up the alt attribute in a small box, it's not meant to work like that (and it only "works" in IE).
The 'alt' attribute should be used so that user-agents who can't/won't (screen readers, or perhaps if the image link is broken) display your image get something that is a textual representation of the image.
If you want a small box popping up you should use 'title' I think, or perhaps 'longdesc'. I don't really know.
Anyway, I don't really know what you want. Perhaps something like
this (hover over the nav menu). Or perhaps something like
this (use mozilla/firefox (or perhaps Opera) and hover over the 'recent entries in the right side bar).
I'm guessing something like the second link is what you want, but Distler's solution doesn't work in IE, which might annoy you if you have many (more than one or two per cent) visitors who use IE. So perhaps you can hack together a combination of both?
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