This sometimes indicates a hard drive is on its way out. It only needs a knock or rough handling to cause problems.
Hard drives are so cheap these days (I heard of someone getting a brand new Maxtor 120GB model for just £45 the other day) that it's not worth risking possibly dubious buys.
Around £50-£65 should get you an OEM 80 to 160GB drive depending on make, model and specifications; Scan, for instance, offers a wide choice and even a 200GB model is £80 or less:
Sometimes even cheaper on its TodayOnly page.
There's also dabs, e-buyer, Novatech etc.