Even if you could overclock it(very unlikely) the cpu cooler wouldn't be able to handle anything that would actually make a noticeable difference in speed.
hay, i aint got a solution, but i got my dell xps 600 to overclock by 100mhz, and it was horribly unstable. i got a Pentium d 950 in my gx620 and if i can overclock it i can rig a new cooling system on it. i will help by showing you the stuff i have used. i havent had any of this softwere damege any of my ps's, but be prepared to crash on clock change. let me know if any of you get anywere.
the overclocking tool that barly worked in my xps was called systool http://www.techpowerup.com/systool/
a ulitily to monitoring varous infermation on your computer, you can use it to check current clock speeds of cpu and ram. https://www.piriform.com/speccy
if you succeed a cpu stability tester, this will check for artifacts in your cpu like how oc scanner will in your gpu.
http://www.ocbase.com/
and befor anyone tells me to by a better motherboard, i got a evga classified sr-2 as my main tower, this mini dell was in the trash and after 60$ on a gpu it became my 15lbs portable gaming rig. i want more cpu power but dont wanna drop 50-200$ on a Pentum d 965.