Quake 4 problem - it jumps to the main menu as soon as i enter the level

I bought a copy of quake 4 when i was over seas for a holiday late last year.

i just installed it now on my pc:
-core 2 duo 1.67ghz
-vista home premium
-1gb ram
-GeForce 8600M GS

it runs very smoothly but when i complete the first level and go on the next, it says loading then jumps to main menu.

I cant play the game any further 8(

im sure its not a demo because the game is on a dvd and is 2.5gb.

any ideas what wrong?

 

i think version 1 maybe i have to update??

 

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