Memory is not disk drive space. It's memory in the computer (also called RAM). And if Lightroom is telling you that it doesn't have enough memory, then that's really the only interpretation, you don't have enough memory.
The easiest thing to do is make sure you don't have other programs open at the same time as you are exporting from Lightroom. If that doesn't fix the problem, then you need to have BOTH a 64-bit operating system and 8GB or more of memory.