My gears started slipping on my Giant Iguana Disc (probably neglected chain) so I've changed chain and cassette. Wanted to change crankset so I've bought a Shimano FC-M311 but found that those 4 teeth in the picture touch with the dérailleur attached to the frame. I'm trying to figure out what crankset to buy but have no clue and can't figure out how to find one!
You have a couple options, but both require new parts.
1) Buy a clamp-on front derailer. Currently it sounds like you have an E-type derailer that mounts between the frame and the bottom bracket cup. Your bike ought to be able to take a standard clamp-on derailer that mounts above the bottom bracket, well away from that part of the crank.
2) Replace your bottom bracket with a longer spindle. This will move the crankset outwards, hopefully clearing the derailer bracket.
Of these two, the easiest and best solution is probably 1. If you want to really do this on the cheap, you could grind away those tabs - they aren't necessary for the function of the crank at all (as far as I'm aware!).