can't drag Thunderbird icon to application folder

When I get to the step in the process where I'm supposed to drag the icon/program (in this case, Thunderbird) to the application folder, it won't let me. It lets me drag, but when I get over the application folder icon, I see a circle with a slash (i.e. "not here")

I have tried:
--deleting the application, download, etc. and starting over
--repairing permissions
--restarting
--clicking the dmg file, then dragging Thunderbird to the Application folder via Finder

I had Thunderbird up and working, just had the "volume mounted" issue (icon showing up in the right of the desktop). I was trying to fix that...

New Macbook, OS X 10.5

Help!
Terri Z

Just to be sure, these are the steps for installing the program:

-Mount the DMG file.
-Once it is mounted, open the disk image from your desktop.
-Find the application icon inside the image and drag it from there to the Applications folder.

Is this what you are doing?

Make sure you're downloading Thunderbird 2.X from this Link.

Next... Check to see if Thunderbird is running. You can't install or re-install if it's active as it won't let you. Thunderbird sometimes hangs when checking for new messages in a newsgroup or mail. Thunderbird needs to be completely shut down.

You should be able to completely uninstall Thunderbird and re-install it. Just make sure you've located all the files and that every process has quit.

Regards.