You can try deleting DM and re-installing from scratch. Every now and then a download is corrupted or the software doesn't install properly.
Are you downloading DM from the BlackBerry web site? That gets you the freshest version. BlackBerry - Desktop Software for PC
Also check how much RAM the computer onto which you're installing the software has. I had an old ThinkPad and when I opened DM on it the delay for the entire program to load could be several minutes. Sometimes it didn't open at all. Turns out I only had 256MB of RAM, the original factory configuration. Once I upgraded to 2GB I stopped having issues with software like Desktop Manager.