Text File does not open with Notepad

Hi All,
I installed a software called ultraEdit on my windows 7 machine, but then after uninstalling it my text documents no longer use Notepad as the default program to open. I tried changing the default program that text documents open with, but it is not letting me. Could anyone give me some helpful hints on what might be the reason behind this behaivior?

Note:
The "always use the selected program to open this kind of file" check box is disabled when trying to do any changes.

I tried 'open with', and I have chosen notepad yes (while the checkbox for 'always use this program for the selected file type' is ticked ), and It opens well.

I would like it to open automatically with notepad without choosing open with.... Any ideas?

When you right-click a txt file and select properties, what program does "open with" point to?

it points to unknown application, and I cannot change it.

I tried the set association in the control panel, but everytime I change the .txt to Notepad it flips back to unknown application. It is not letting me change it.

I had recently installed a video-playing program. When it proved buggy, I uninstalled it using revouninstaller aggressively. When I double-checked through the registry, the program had ingratiated itself into every video format/extension entry where I couldn't delete it. While I didn't notice if I was hampered by those registry entries, ultimately, I had to revert to a restore point prior to its install. Could your case with ultraEdit be similar? Perhaps an earlier restore point may solve this problem?