My car alarm goes off when i open my bonnet any idea why this is happening 1998 toyota avensis

That sound alarms you not to leave your keys behind because of multiple security reasons.

Reprogram your cars alarm,there are many ways depending on your alarm and car model

toyota avensis 1.8 1998 on starting car it runs for a few seconds everytime then stops,this happens every time i try to start it wont keep running

There is none except the Toyota dealer or a well equipped Locksmith. Sorry you are just stuck here.

check heater tap under bonnet and make sure it opens when controls set to hot and engine running

I would recommend checking the hood switch. Raise the hood and lock on the left or right strut tower area. You should see a small switch with a rod that sticks up.try disconnecting the switch and see if the problem goes away.If not you have an interior sensor on the headliner under a trim cover. The cover is clipped on.pull the cover down and try disconnecting the sensor to see if it fixes it.

Check the fuses. Because of frozen locks, the motors could not operate, thus overloading the circuit and blowing the fuse.
Hope this helps

somehow the car when on valet mode, go to a alarm shop , they should be able to re arm your car without to much damage to your pocket, or read your cars manual, it might inform you on how to fix the problem.

Not a solution just another problem...Lost my car fob key, have a replacement key no circuit. As I open door, sets off immobiliser like above for20 mins. I had someone look under the glove compartment but that got me no joy. How can I (non techy) resolve this so I can drive my baby home...Vreg Avensis1.8 model?
many thanks

Check the small alarm switch under the bonnet is closed tight when the bonnets shut - put some tape on the bonnet to firm it up .