Any of them, but slackware is the most complete Ive ever tried, simplest to deal with too.Its not all GUI driven setup like alot of distros nor does it have dependency checking for software, but that also makes it less prone to get broken and take less space on your hard drive.However, with such a small hard drive and having Im assuming windows on there too I'd seriously recommend getting another drive, the old used small ones are quite cheap, maybe $10-$15 these days.Most of the newer linux distros are big, very very big.You may want to try an older version of some distros.Xfree86 Xserver version 3.3 is way smaller than version 4.x for example.Your going to run into space issues if you use slackware 9 or 10 for example.
Another choice just for learning is try knoppix or any other live CD distro, these are linux systems on a CD, no hard drive needed, they run in ram, and usually have provisions for installing to a hard drive too.
go here and surf for awhile, read alot first
http://www.linux.org