The easiest fix would be to uninstall Blackberry Desktop Manager, restart the computer and then reinstall Desktop Manager. If you do not want to do that then try the following fix. I found this in one of the Blackberry forums and it worked for me.
Complete the following to fix:
1. Make sure BlackBerry Desktop Manager is closed.
2. Locate the Intellisync folder on your computer. This folder can be found at C:\Documents and Settings\<user name>\Application Data\Research In Motion\BlackBerry\. If you can't find the Application Data folder, see note below.
3. Rename the Intellisync folder (e.g., Intellisync_old).
4. Open BlackBerry Desktop Manager.
5. Connect your BlackBerry device to the computer running BlackBerry Desktop Manager.
6. If you are running BlackBerry Desktop Software 4.2 or later, double-click Synchronize, click the Configuration tab, and then click Configure synch. If you are running BlackBerry DesktopSoftware 4.1 or earlier, double-click Intellisync, and then click Configure PIM.
7. Configure your synchronization options. For assistance, click Help on the Device Configuration window.
8. When have finished configuring the synchronization options, click OK to close the Device Configuration window.
9. If you are running BlackBerry Desktop Software 4.2 or later, click the Synchronize tab, make sure the Synchronize organize data check box is selected, and then click Synchronize now. If you are running BlackBerry Desktop Software 4.1 or earlier, make sure the Synchronize PIM check box is selected, and then click Synchronize now.
A new Intellisync folder is created.
If the Application Data folder cannot be located, the folder may be hidden. To make this folder visible, complete these steps:
1. While viewing the folder C:\Documents and Settings\<user name>, click Tools > Folder Options.
2. Click on the View tab.
3. Select the Show hidden files and folders option under Hidden files and folders.
4. Click OK.