When I've got an old hard disk I want to re-use, I use this program
before installing anything.
http://hddguru.com/software/2005.10.02-MHDD/You can put MHDD on a bootable CD or even a bootable floppy, and use it to test and erase hard disks.
There is a bit of a learning curve in using it (it's all command line) but I have found it very useful - it can make the disk run faster! Caution is advised, especially if you have more than 1 disk in your computer, as it would be very easy to accidentally wipe the wrong disk. I
always disconnect every disk except the one I want to work on.
I tend to :
boot MHDD, and scan the disk. I note down the number of sectors that take <5ms <10ms <50ms etc to read.
I then run the erase command.
I then scan again, note the results and usually see a reduction in the slower to access sectors, and an increase in faster ones. If any sectors were unreadable, chances are they'll be readable now!
Finally I use the built-in fdisk program, which makes a simple single partition on the disk.
I then reboot with my OS install disk (eg Windows XP) and off you go with a clean install, and a disk that's been completely erased.
It's not a security disk eraser, someone with enough time could probably retrieve the data, but every sector
has been erased, so it wouldn't be easy. And you've got a thoroughly tested disk in the bargain!
That said, if you're already re-installing Windows, you're a bit beyond this point now, unless you want to start again
Ian