If this is a new version number, from 5.3 to 5.4 for example, then you must 1st completely deinstall the previous STEP7. The installation routine normally starts with this automatically.
The problem may be that the previous deinstallation wasnt 100% succesful. Maybe it was interrupted before it was finished. If that is the case, you may have to remove all Siemens entries in the registry manually.
You can look for "complete deinstallation" on Siemens Support site.