This actually isn't a problem with Sublime text, it's a generic quirk introduced in Vista and left lingering in W7. I actually recently made a blog post about this very issue, you can find it at anders.tonfeldt.se/?p=775 but I'll detail it here since it's generally considered bad form to drop an url and leave.
- Open regedit.
- Navigate to HKEY_CLASSES_ROOT\Applications\sublime_text.exe\shell\open\command
- Verify that the path is accurate, correct it if it is not. Exit regedit.
- Open task manager via ctrl+alt+del, kill explorer.exe, go to run and type explorer.exe (or skip this step and simply reboot).
- Now attempt the same thing, right click a text file, open with, navigate to sublime, and it should now appear in the list of available applications.
My post above uses another application as an example and it's not guaranteed to fix your problem, but it's likely to be the same as I've encountered it countless times when installing / uninstalling / reinstalling an application, or just updating one, in Win7/Vista.