dumb crank question

i put new cranks on my bike. they seemed to be on nice and tight but they work themselves loose in about 2 miles. one thing to consider is that the new cranks came with crank bolts that go with bottom brackets that have a hole in them for a bolt. my current bottom bracket is lol shitty and uses nuts. i put the old crank nuts back on with the new cranks. does this mean:

(a) i cannot use these cranks with this bottom bracket, and i need a new bottom bracket too?;
(b) my crank nuts are just worn and i need new ones?; or
(c) i am a moron?

i was actually going to get a new bottom bracket anyway because i like taking things apart, etc. but i didn't want to spend money on tools (ok i do want more tools but i was trying to be responsible with what little money i do have)

New crank nuts would be a good idea (and they're cheap too) but if it's a square-ended BB the cranks are going on, then they can work loose regardless of wether it's a nut or a bolt holding them on. You need a long-handled spanner with a lot of leverage to do them up tight, and it's usually worth rechecking them after a week and retightening them. They need to be REALLY tight.

ok thanks, i'm going to try that. i was worried if i pulled it too tight i would cause something bad to happen (like stripping) if the problem was things not fitting together right. this didn't happen with the old cranks--there was nothing wrong with them, only that one of them came from my other bike, which i want to put back together eventually--but the nut did look worn on the bottom, so we'll see.