Applications won't open, but are shown to be in Task Manager.

I have Windows XP SP2.
I just did a clean install a couple weeks ago, but there are still some of the same problems.

I sometimes attempt to open applications, but they don't. However, task manager shows they have opened.
This happens most often with Mozilla Firefox and Thunderbird, which I use for email.
Then the processes can't be closed in task manager.
Also, when I attempt to shut down or restart the pc, the applications, such as Firefox or Thunderbird, will sometimes, suddenly open and function correctly.

Another issue I'm having is, there are no users shown after clicking on the 'users' tab in task manager.

Please let me know if you have any ideas.

Thanks, Rob

 

http://www.winhelponline.com/articles/44/1/Why-is-the-Task-Manager-quotUsersquot-tab-empty.html