Happy now?
You can choose the interval (in milliseconds!). The last one you used is saved in the registry (at HKEY_CURRENT_USER\Software\Segosa\F5 presser) when you close the program and read when you reopen it also.
The F5 key will never be sent if the application's own window is the active one so don't worry about that.
The 'U' key is to update it whilst it's already enabled. If it's disabled and you change the interval then you don't need to press it.
EDIT: wait, there seems to be a bug..
EDIT: fixed...
EDIT:
source, including the project files.