Overclocking: daft question...how is this possible?

I run a sempron 2400 and, since my mate (fixes pcs) installed it with me watching, we never fiddled with any jumper settings.

It seems that LW runs helluva fast (some mp3s download at incredible speed compared to my original HDD).

This "tester" hdd was reformatted remember!

I do not want rid of LW...no way!

how can you tell if a cpu is overclocked?