Open With Arachnophilia:
This feature, located near the bottom of the Tools menu, is something that you'll likely only use once. It allows you to add an "Open with Arach" option to the context menu for certain file types in Windows Explorer. That is, when you right click on one of the specified files, and "Open With Arachnophila" option will appear on the menu.
As you can see in the screen shot above, I have chosen to do this for HTML files, but there are many types available. Unlike many program which simply take over your file associations, at least Arach gives you a choice about it.
One thing to keep in mind, while this option can create file associations to Arach for you, it can not disable them if you later change your mind. If you wish to clear associations made with this option, you'll have to do it the old fashioned way by going to View | Folder Options | File Types in Windows Explorer and manually editing them...