Resolving the problem
IF you are experiencing this behavior when installing the full IBM SPSS Statistics program, THEN you must first uninstall any instances of the SPSS Statistics Trial/Demo version (if present). It is also recommended that you rename the previous installation folder. For example, Statistics21 to Statistics21_old. Once this has been done, you should be able to install the 'full' version of IBM SPSS Statistics normally.
IF you are experiencing this behavior when installing IBM SPSS Statistics Fix Pack, THEN the cause is most likely due to a mis-match between the bit-version (i.e. 32-bit, 64-bit) of your Statistics program and that of the fix pack. Confirm whether you have SPSS Statistics 32-bit or 64-bit installed and download the fix pack with the corresponding bit-version.