I think the magic number is 7, if you erase everything 7 times that data is no longer retrieveable.

One way id to look for hard drive erasers on download.com.

If you wish to just reformat and reinstall, lets assume its Windows XP that you are reinstalling.
1. Put Windows XP into your CD-ROM, restart  your PC.

2. When the installation disk finishes booting, follow the instructions from there to reformat your hard disk and reinstall windows.

An even better way would be to install Linux!