You must have either a too long sample or too many samples already loaded for the ram that you've got installed, i'm guessing you've only got the 16mb ram. Either chop the sample up manually into smaller bits, then slice these smaller bits individually and delete the parts you don't need (to reduce memory use), or load up less samples if that is the problem, or best of all buy the 128 mb ram expansion, that will allow you to slice anything within reason.