Never heard about RANDBETWEEN - sounds like a UDF based on RAND to me? I doesn't exist on any of my machines (2000, XP) anyway.
However, you can use =RAND(), which selects a random number between 0 and 1. To get it in the correct range / format, multiply with a number of your choice, and combine with ROUND (or ROUNDUP, ROUNDDOWN).
Example:
=roundup(RAND()*100)
for a number between 1-100.
// Patrik