What are the differences between AT and ATX cases?

I've got an old midtower case that housed a P90. It still seems like a solid case. I was wondering if I could put an Abit Bh6 board in it. Will this work? If not, is there any modification I can make to make it work?

Thanks in advance....

 

[LongTimePCUser]

You won't be able to use an ATX system board in an older AT case.
ATX system boards have different power connectors than AT system boards.

 

[Kp99]

With ATX the MB controls the PS, with AT you have a switch going into the PS. Cant use you AT case on an ATX MB, just wont fit.