If it's a Corsa B (which I believe it is ... 1998) then yes you do definately need an ISO adaptor.
The standard stock radio/cassette is pretty easy to remove, it's got some allen key (hex) bolts over the release holes (2 on each side) so you'll need to remove them before you can get at the hole itself. A generic pair of radio removers will do it, otherwise, a few very thin + long knitting needles/screwdrivers...even then I'm not too sure.
I can imagine it being a right female dog to remove if you dont have the right tools to get down them holes.
After its out, it's a piece of cake.