Also there are times that you can delete the Dreamweaver cache file. You'll want to exit Dreamweaver and your extension manager. Then Navigate to your Dreamweaver Configuration folder. Once there you'll want to delete your WinFileCache File. It will have a series of numbers and letters at the end of the filename.
I use WinXP and Dreamweaver CS3.
C:\Documents and Settings\<user>\Application Data\Adobe\Dreamweaver 9\Configuration
Once you do that, restart Dreamweaver, and then exit Dreamweaver and then try installing it again.