I don't remember where I found this, but you can modify your registry to get this working. I've tested it with both TextPad 5 & 6, and it works.
Here's what regedit exports:
Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Applications\TextPad.exe\shell\open\ddeexec] @="[open(\"%1\")]" [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Applications\TextPad.exe\shell\open\ddeexec\application] @="TextPad.6" [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Applications\TextPad.exe\shell\open\ddeexec\IfExec] [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Applications\TextPad.exe\shell\open\ddeexec\topic] @="System"
You should be able to import the above if you save it as a .reg file, or you can do it manually. If you're using TextPad 5, you need to change the value of the "application" key to "TextPad.5.0"