Yea you can, but your oc will be limited, you can use clock-gen and try to overclock. The reason your oc will be limited is b/c you can tweak your oc like raise or lower the v-core or tighten timings.http://www.cpuid.com/clockgen.php
Yes, surely you can overclock your pre-built computer using a software. However, do know that the computer can become corrupted. I tried clockgen on my current system once, and I almost jacked up the computer. It started acting crazy.