Thanks HS for bug reporting and great suggestions :)

What's new in version 3.2.3:

1. New: Check for Updates command.
2. Fix: Compare MD5 dialog should recognize lower case characters.
3. Fix: Combo boxes should not be fix width.
4. Update: Separate Unknown category into Unknown & Loaded.
5. Update: Add *.zip & *.rar by default.

Regarding to #4, otkin asked how to check only new files (files have never been checked), I use this update to make this process easier:

1. Add the containing folder into Md5Checker WITHOUT auto starting
2. Switch to the Loaded category
3. Remove all listed items <- Now only new files are left.
4. Start checking

But I'm still thinking about a one-click solution, maybe it will be implemented in the next version.