I'm assuming you have a Guard II Alarm system, so this information only applies to that.
You can reset the immobiliser by unlocking the car with the fob once it is activated. If this doesn't work then it is likely you have a faulty alarm module (happens if the battery in the car dies for a few weeks, or just from age!). You can pull fuses to permanently disable the immobiliser, there is a certain order to it which can be seen here:
Click
Note this will not disable your alarm or central locking, just your immobiliser. If you want your immobilser back you will need to find a used (or new if you're very lucky) alarm relay, which you will then need to recode your fobs for.