I don't have any experience overclocking brand name boxes, but I suspect that they are deliberately not overclock friendly, i.e. the BIOS may not allow the necessary changes, or may greatly restrict them. Overclockers build systems around motherboards that are designed with overclocking in mind, and allow fine adjustments of a number of different parameters.
Anyway, the first thing we need to know is exactly what model processor is in your computer? At 3GHz, I'm guessing it's an Intel Pentium. For Intel processors you can use the Intel Processor Identification Utility from
hereOut of interest, I have a second box with a 3GHz P4 540 running easily at 3.44GHz. It has an air cooler, but it is a more efficient cooler than the stock cooler.