[andrewas]

Emulators are illegal if:

You dont own the BIOS to the original console. The emulator needs an image of the consoles BIOS chip in order to work, if you dont own a console you cant legaly download it and therefore cant ever make the emulator work. Fortunatley, several consoles have had their BIOS'es released into the public domain, but forget about sony every realeasing the PSX bios. Ever.

You dont own the original game, although there are several developers who have released their games into the public domain again, so check around. If in doubt, email the developer (if they still exist, otherwise find whoever has the rights) and ask.

If you have a legal BIOS and a legal copy of a game, its legal to emulate it. It is generaly assumed that it is legal to download an image of a game if you hold the cartridge, since reading a console cartridge takes significant equipment.

At least thats what its like under US law, and british as well. AFAIK.