I had exactly the same error message when installing version 4.3 of the software. I resolved the issue by installing Microsoft Windows Script 5.7 for Microsoft Windows 2000 and Windows XP (which only took a few minutes). I have Windows XP as well.

For more info go to the Blackberry Technical Solution Center and search for 'BlackBerry Desktop Manager Installation Error 1720'. The following instruction was useful:

The following are possible resolutions for this issue.

Cause 1
From the Microsoft Support Knowledge Base Download Center, download and install Microsoft Windows Script 5.7 for Microsoft Windows 2000 and Windows XP.

Then, click on Download Center link and download the script. You will then need to restart you computer and start installing the Blackberry desktop software again.

you could also run another program from microsoft to cleanup the program installed that halted,
http://support.microsoft.com/kb/290301

go there and use that program, it might fix it