I bought a cheapo one of eBay a couple of years ago and it immediately wiped the os and everything on the hd. Quite a few people said this was impossible but a bit of internet digging suggested that a slight change of power could effect the bios and do exactly that.
I wasn't overly concerned as I was running Linux so just did a re-install of the latest version from a magazine freebie CD - could have been costly if it was windows though as I'd long since list the discs etc.. Personally I'd recommend saving everything and having a back up os disc.
Battery was fine afterwards though!