Well, if you look at the motherboard and have the ports (mouse, USB etc) on in the upper left corner, the PCI ports below (either 4 or 7 of them, i'll return to this later) and the CPU somewhere in the middle, you most likely have an ATX card.
Now, depending on the amount of PCI ports (including AGP and PCI-E ports if there are any) you could have mini-ATX or full sized ATX.
If it's not anything like this, please try to illustrate what it looks like.
Ex of what I'm talking about:

If your motherboard looks something like that (note the positioning of the left-most parts, the others doesn't really matter), it's ATX, if not it's most likely ITX or BTX (the latter is true if you have new-ish DELL, the former if it's a very small computer (or barebone)
Now, if it
IS ATX you won't have any trouble at all finding a case, since it's still the most widely used mobo standard around. if not, you might have more trouble.
Now, who made your PC? is it an OEM computer? (Compaq, HP, Fujitsu Siemens, DELL etc), or was it a smaller local computer store or similar? See, if it's an OEM machine, you might have trouble actually getting teh motheboard out of the old case, since they like to glue the board onto the spacers and other crazy stuff to prevent people from doing stuff like this.