hp compaq blue screen

hi guys, my hp compaq 6735s boots to a blue screen saying irql not less or equal with some memory address numbers on the bottom but i cannot read them i have 1 stick (1gb) of mem in. could it be that the mem is knackered? or a virus or maybe something else? i have ran diagnostics an it says mem passed but cannot find the hdd
any help much appreciated. by the way it is less than a year old and running vista