If you downloaded the cd/dvd iso then be sure to check the md5sum for the original iso. From the cli;
~#cd /downloadisolocation #cdromiso.iso cdromiso.md5
~#md5sum -c cdromiso.md5 #substitute the correct name to check
This will check the download iso with the known md5sum that you also get with the iso.
You could have a bad burn, to check the cd/dvd with the original iso md5, use this CdromMd5sumsAfterBurning
If you are using M$ Windows then look at 'md5sum.exe
These links and others can be found at 'Slackware-Links
'. More than just SlackwareŽ links!