Question about disappearing program in the "Open with" dialogue

[jimdagys]

I normally play videos by right clicking on the video and choosing "Open with" and then choosing Media Player Classic. (See screenshot). However, I was messing around with the computer, installing some programs that I shouldn't have, and then Media Player Classic disappeared from the "Open with" dialogue list. I then clicked "Choose program" and then "Browse", and browsed for Media Player Classic (exe on the desktop), but I couldn't add Media Player Classic to the list. Then I found a program called OpenWithAdd that solved the problem (allow me to add Media Player Classic to the "Open with" list).

OpenWithAdd utility helps you register an application to the Open with
dialog in Windows XP. This utility can be helpful in situations where
the Open With dialog does not list an application even though you browse
and locate the application.
http://windowsxp.mvps.org/openwithadd.htm

and this solved the problem.
My question is:
1) Why did Media Player Classic disappear from the "Open with" program list?
2) Why couldn't I add Media Player Classic when I tried to add it to the list (choose program> Browse)?
3) How did the program OpenWithAdd allow me to solve this problem (add Media Player Classic to the "Open with" list)?

[johns0]

When you installed/uninstalled a program it removed a registry setting for media player classic,OpenWithAdd restored the registry setting that was removed.

[jimdagys]

Also noticed that about 20 programs shortcuts were removed from my Start> All Programs menu. The programs were still installed, but I had to put the shortcuts back in the C/Documents and Settings/All Users/Start Menu/Programs to get them to appear back in the program list. Some program icon shortcuts were also remove from my desktop. Now, several days later, I haven't noticed any other problems.