Are you trying to upgrade or install fresh? If you're trying to install Slackware 12.1 fresh, make sure you format your partitions -- you could be getting some interference with old packages from your previous 12.0 installation. Also, you should check the md5sum of the CDs/DVD that you burned to make sure the CD is good. Try this script (from Alien Bob, of course) for that:
http://www.slackware.com/~alien/tool...o_md5_check.sh (also make sure the md5sum of the ISO matches the md5sum on the Slackware site/mirror).
If you ARE trying to upgrade, you'll need to provide us with way more information (like every step you took).
As a final thought, be sure to read CHANGES_AND_HINTS.TXT and UPGRADE.TXT on the first CD/DVD (and the rest of the text files too, of course) regardless of whether you're installing fresh or upgrading.