It sounds to me as though you will need to do a manual uninstall of the previous version on those workstations. This does happen occasionally when deploying SEP over the top of much older versions (i.e., SAV 9 or 10). You can try a simple uninstall on those workstations using Add/Remove Programs, but it may not work.
Probably the easiest way to proceed would be to use the Symantec CleanWipe utility to wipe anything left behind by the previous installation and the attempt to install the new SEP. You have to contact Symantec support in order to get a link to download Cleanwipe, unfortunately.
You can also try the manual uninstallation process, but it takes a lot longer than using CleanWipe. You can find the instructions for the manual uninstall procedure by checking this document for your current (old) version that you need to uninstall:
http://service1.symantec.com/SUPPORT/ent-security.nsf/docid/2002031914291648