When I start engine the word SECURE is displayed on the computer screen. The radio will not work but engine starts etc etc

rough guess is that the battery is faulty
have a load test done on it
cold weather kills failing batteries quickly

The check charging light should be on if the engine is not running.
The most common cause is a bad chip in the key. Have you tried a spare key ?

If you spray starting fluid into the air intake of the engine does it start? If so check you fuel pump circuit.

It might be that the original key has a "chip" in it. This key sends an electronic signal to the computer that allows the system to "start". Visit your Saturn dealer (if you can find one) and get an original key for it if you lost the original one. You might try Ace Hardware, they sell "chip" keys; make sure you know your make and model, VIN, etc.

These vehicles use Fords Securilock transponder system to prevent vehicle theft. Basically, there is a little radio chip in the key that transmits a code to the computer when you start the car. If this code doesn't match the computer, the computer disables fuel and spark so the engine won't run. If you have a spare key, try that to see if it works. If it doesn't work, then the Securilock system needs service. Unfortunately only the dealer can service this system because it is integrated into the vehicle computer.

If your spare key works, then the key you tried to use has a damaged transponder and the key needs to be replaced. If you don't have a spare key, contact a locksmith and have them make you a new key. I have to warn you, transponder keys are not cheap. They can cost easily $75 and some even go for $150. But, that is probably cheaper then having Ford make one and even if the spare doesn't work, when you get the system fixed by the dealer they can program your spare key as well so you don't waste the money on the key.

try the spare key/remote.

YOU NEED TO REPROGRAM IT

You checked all fuses? Dis-connect positive battery lead for a few seconds and reconnect. Do you have spark & fuel?

Sounds like an antitheft problem I would start with the antitheft system. Usually when you only have one key all keys have to be programmed together even the original. When you have more than one it is easy to program additional keys.