To get it installed and working, try the following:
1) Install it using the Right Mouse Click > Select "Run as administrator" (Vista/Win7 users), select Repair from the options (Repair or Remove are the options).
2) Turn of your Anti Virus software (especially if you are using the latest version of Avast free, it will not allow the file to install/uninstall correctly) it might be preventing you from installing it correctly.

If doing the above doesn't work and you are still having problems, try the following
1) Do an OFFLINE activation (the installer guides you through it, so it's easy to do)
2) Do the FULL uninstall as described the KB #685 (this is the last ditch effort to get it installed correctly and requires removing the CS767 entry in the Windows Registry)

Mark