open your registry (regedit)
Go to HKEY_CLASSES_ROOT\http\shell\open\command
You should see one key, named (Default). Double click this to get an editing popup.
At the end of the Value data: field you should see the text -- "%1". (for opening in a tab)
If you see --new-window "%1", delete new-window part
you must have
"C:\Program Files\SRWare Iron\iron.exe" -- "%1"
repeat the same for