Michael, thanks for the link.
First let me mention that I uninstalled adobe reader and everything went back to normal except - of course - I couldn't open a .pdf file which was not good.
I reinstalled adobe reader hoping that the new install will come clean but once done I was back with the problem.
The link you sent me lists several solutions one of which is to perform a registry fix which I wouldn't attempt because - as it mentions - I could end up with more serious issues.
Another solution it mentions is to set the default program to a web browser, email client... etc. This is simple to do but could you please explain to me what this will do? Let's say I set the default program to internet explorer. Will it stay as the default program forever? Will it open a word document, or corel draw for example?