Read manual (http://files.canon-europe.com/files/soft31340/manual/CUG_EOS450D_EN_Flat.pdf) if you have it - page 114 :)
This is for reverting all the camera’s shooting settings and Custom Functions to
the default settings. This works in the <P> and other Creative Zone modes.
1 Select [Clear settings].
�� Under the [spanner thing] tab, select [Clear
settings], then press <SET>.
2 Select the desired setting.
�� To revert the shooting settings to the
default, select [Clear all camera
settings], then press <SET>.
�� To revert the Custom Functions to the
default settings, select [Clear all Custom
Func. (C.Fn)], then press <SET>.
3 Select [OK].
�� Select [OK], then press <SET> to clear
the camera settings.
�� When [Clear all camera settings] is
set, the camera will be reset as
shown in the table below.
(SNIPPED)
I'm only familiar with 20D and getting familiar with 550D with regards to DSLRs but the "clear settings" function in the menu should reset it to all to default. Also, what lenses are you using? does the 450 have an Error 99 code? Have you tried to reflash the firmware without knowing what you were doing?
there will be plenty of people with knowledge to help you :) I shouldn't think you're the only person in Caithness to own a 450 :D