2D,
I would suggest that it is not your HD that is in need of replacement, but your O/S! Windows ME really is a pile of p00!!
Others on this forum espouse Linux and / or Macs, but assuming you wish to stay with MS, with XP you can have separate, restricted, accounts for the "pups" where they can play to their heart's content without breaking your system.
It would also be a LOT more stable and have much better support for peripherals - although not all Win9X progs will run correctly under a proper 32-bit, pre-emptive, mutithreaded, protected-mode, multi-tasking O/S like XP!
But for security's sake, do make sure that your important files are backed up somehow - in case your hard disk DOES fail!
And you should always do so anyway if upgrading the O/S - just in case!