Linux

I have a friend's computer up here, it's an old one. A Pentium running at around 120Mhz, 64 Megs of Ram PC-100 if that, I can't tell because the ram sticks are under the power supply and I didn't really want to take it apart. It's got at least 2 Gigs of hard drive on it. He's not worried about getting it back soon aparently. So I'm asking, which version of Linux should I put on it? I want to see how it runs on an old computer. Thanks