how to sweep hard disk of confidential data

["H. S." <greatexcalibur@yahoo.com>]

I am in a situation where a computer is to be given away and prior to 
that it's hard disk needs to *thoroughly* cleaned (data consists of 
financial information, reports, class exams, competition exams, etc).

One method I have found on google is:
$>dd if=/dev/zero of=/dev/hda
to clean the first disk. Now this only writes data once. I am not sure 
about this method's thoroughness since I have read that multiple writes 
are necessary to really overwrite the original data.

The second method that I have read is to delete everything from the disk 
and then write a large file (how to get his? random data?) repeatedly 
and fill the disk. Then remove these files with 'wipe'.

Any suggestions? Or any alternate methods? Then there is also the option 
of using a Windows programs to do this. But I am familiar with those.

thanks
->HS