there's lots of places to go and find out about overclocking now, without advertising it here.
Overclocking if fine and dandy as long as the user understands that not all overclocks work. It may cost him or her money to replace the components and time lost restoring lost data.
Most oem machines are darn near impossible to overclock, they don't have the right hardware. All the overclockers I know build their own machines, so they know exactly what components they have and what they can take.
Overclocking is risk taking, not support or repair.
What happens on a bad overclock
1. System won't boot period
2. Data corruption on hard drive due to heat and overclocked bus
3. Burnt out video cards. Hard drives with bad segments
4. Burnt out cpu's
5. Burnt out motherboards.
6. Numerous hangs and crashes due to heat or bus speeds.
etc
etc
etc
Seen plenty of it, I overclock but understand it could go bad.
My opinion, before I ever overclocked I did a lot of reading and browsing getting to know the process and what works and what doesn't. I built the machine with the best parts available at the time.