I can't open AHSS's directly in Photoshop5. Might be fixed in newer versions. Something with the way it's saved (32 bit maybe??). I have to "edit" the bitmap (brings up MS paint) ctrl A, edit menu > copy to... and save as a new file (24bit). Then PS will let me open it.
It takes a bit of time, as you can imagine, if you have a lot of files you want to edit. I usually leave my SS's as the default bitmap -- I rename them of course -- until I need to edit them.