If teh system was working perfectly before and the only thign you have changed is teh CPU,the options are rather limited:
1 the CPU
2 they are in fact not compatible
3 and this one is at a strech, the PSU can cope with the e6300 but not teh Q8300.
There maybe others but they are what first come to mind.
If it was me personally i would eitehr try the Q8300 on another motherboard or another Q8300 on this motherboard just to find out whethe rit is working or not, as that is the only component changed. just noticed you said this"All the other components work properly as i have tested them individually." does this mean you have tested the Q8300 as well?