Every time I open Desktop Manager, it ran Windows Installer and gives me this message:

"The feature you are trying to use is on a network resource that is unavailable. Click OK to try again or enter an alternate path to a folder containing the installation package 'BlackBerry 7520.msi' in the box below."

I cannot locate this file within my hard drive nor in the installation disk.
Uninstalling and reinstalling the desktop software did not work either.
Any advise?

are you using the software that came with the device? It sounds like the wrong driver is associated or that an old blackberry that may have been used on the system is still trying to take precendence over the Blackberry you currently have.

My suggestion would be to completely remove the program via the add/remove programs under control panel. After that make sure you delete the RIM folder from the Program Files folder, the common files folder, and any other location RIM folders are created such as local settings and application data as it may be attempting to read something from one of those folders.

If you have the original software, once you remove the above, if you have the ability to, try downloading desktop manager from the Blackberry Website.

Your PC thinks that the Desktop Manager in not installed or not completely installed, so it's looking for the .msi file to either install it or complete the install.

Do you have the install program on cd or your hard drive?

Try this. Search for and download a free tool called CC Cleaner. After you install it, uninstall the desktop manager and restart your PC.

When it comes back up, open up CC Cleaner, click on the icon for the registry on the left column, and after it scans the registry, see if there are any entries for Desktop Manager. If there are, delete them, and try to reinstall the DM.

Are you using Vista by any chance?