Do you know if your car has PATS (where you have the red chip in the key)? If its a T25 then it probably has, although some early ones do exist without it. Older ones tend to have some awful wipac thing in the centre console - and they tend to be a whole lot of grief when they go wrong. PATS is integrated into the ECU so is unlikely to be at fault. Wipac is wired in with damp string and they do go wrong fairly regularly which is a bundle of laughs.

The simplest way to reset the ECU is to disconnect the battery for a few minutes. But I'd use the fault code reader first otherwise any errors it has stored will be erased and then you'll be none the wiser!