There are steps you can do to clean up your previous 6.4.1 SDP installation:

1) If you don't have other QNX SDP version installed, you can remove a file called "vpd.properties". For Windows you can
 find it under <WINDOWS_SYSTEM_ROOT>/vpd.properties, for Linux you can find it under <ROOT_HOME_DIR>/vpd.properties

2) If you have other QNX SDP version installed and you want to keep those installation meta-data, you have to search for
 string "6.4.1" in the vpd.properties file and delete all lines with that string.

Last thing although not required, check you "QNX_HOST" and "QNX_TARGET" environment variables. If  they point to "6.4.1"
 then either change them or remove the two variables from your environment.

If the above steps do not fix your problem, please attach your installation log file to this post. You can find the log 
file under the root directory of your system drive (for Windows C:\, for Linux /)

BTW why do you need install a previous SDP?

HTH,
Andy