This drove me nuts for awhile. Here is what worked for me.
1. Find your Blackberry Desktop installation package on your HDD. Note that it may not say anything about "Blackberry". In my case it was something like 501_bo49_multilanguage.exe if I remember correctly.
2. Put it into it's own folder.
3. Go to the folder and right click the file (do not double click). Select Winzip (you need to have winzip installed on your computer). Select "extract here". It'll take a few minutes.
4. Re-start your PC if you're stopped the windows install process. If it's still showing you the error message that it can't find Roxio Media Manager.msi, select browse. Browse to the folder you just created and double click Roxio Media Manager.msi. It should complete installation and you'll get a message that you need to re-boot your system.
5. Re-boot and you should be good to go. It worked for me.