That happens when values are left behind by the uninstall (not common). Open a cmd.exe and check as follows (choose the right line based on your PC's bitness):
64-bit: reg query "hklm\software\wow6432node\ibm\client access\currentversion"
32-bit: reg query "hklm\software\ibm\client access\currentversion"
If IBM i Access for Windows is not installed then the key should not be found. In this user's case it will be found, and they need to go trash it. One does that either via regedit and navigate to the key or from a fully-elevated (eg right click -> Run as Administrator) cmd prompt:
64-bit: reg delete "hklm\software\wow6432node\ibm\client access\currentversion" /f
32-bit: reg delete "hklm\software\ibm\client access\currentversion" /f