Left mouse button acting weird

Hi, I have an issue with my left mouse button action. Some setting seems to be messed up. These are the symptoms:
- if I double click on an icon on the desktop, let's say "Word 2003", instead of running Word nothing happens (besides the icon being marked active). If I then hit the enter key Word opens up.
- if I mark some text in a window - such as this text editor - a single left mouse click always automatically selects a whole word (instead of just putting the cursor where the mouse pointer is)
- certain radio buttons and check boxes in web pages don't seem to be selectable at all using the left mouse button (or any button for that matter)

It all seems to me that there is something messed up with the setup of the left mouse button action. However I tried the usual suspects (Folder options, Internet Explorer settings, mouse setup) and nothing so far worked. I tried changing the folder options to "single click to open an item", which helped to open items on the desktop, but did not fix some of the other strange behavior.

Thanks in advance for any help.
Marc