Its a checksum of the disc to make sure the download wasn't corrupted. in the download dir on the server there will have been a md5 file with the same name as the ISO. Download that and use MD5sum for windows to check it out. Make sure the numbers match exactly. If they don't you will need to redownload.
Here's the file and install instructions:
http://www.etree.org/md5com.html
Switching CDROMs might help. Sometimes installers can be quirky, but you will be able to put that one back in once its installed if that is the problem.
I wouldn't recommend Gentoo for a new user the initial setup isn't very nice, and it doesn't "download its packages from the web." It downloads the source code (much bigger file) and compiles it (takes much longer, esp for things like X11 and Qt) That is only any use if you know what compiler flags you want to set and want to do some hard tuning of the system, otherwise you're wasting your time.
Maybe debian if you want to go down that route, but again, the initial setup isn't very nice. Slackware works fine though. This problem must be fixable.