Start > Type Regedit
Click the > Next to HKey_Classes_Root
You'll see Open With List
There should be 3 Microsoft Text Editors
Notepad.exe
Wordpad.exe
Winpad.exe
If you're not familiar with Windows Registry, I don't advise changing anything without performing a backup of the files.
http://windows.microsoft.com/en-US/windows7/Back-up-the-registry
If you're seeing Open With twice then I'd advise you to boot into Safe Mode with Networking and do a scan for malware as advised earlier. System File Checker does not look for viruses and any anti-virus software you have on your system was compromised and unable to detect it. Boot into Safe Mode with Networking - Starts Windows in safe mode and includes the network drivers and services needed to access the Internet or other computers on your network.
http://www.microsoft.com/security/scanner/en-us/default.aspx