of course, there are government and other sources that can, for a price, read the data off the disk no matter how many times you write over it...
So! either (a) don't commit super-sensitive data to machines that can be stolen or impounded - or - (b) encrypt all sensitive data onto separate data drives, using the most secure encryption routines you can afford...
Neither is perfect; but if you have secrets, they have a value in respect to time...
If you want to prevent, say, your secret terrorist plans from being captured by the CIA, you might as well forget it... If you want to prevent your porn from getting you incarcerated, ditto... The government of most countries have ways to make your computer sing like Loretta Lynn...
OTOH, if you want to prevent "ABC Corporation" from reading your secret formula for making cheaper left-handed grotomizying hydrostators or some other high-tech widget, you can feel relatively safe because your competition doesn't have the time, money, or expertise to decrypt--it would be easier to reverse-engineer one...
Good luck!!
Setnaffa is an MCP-W2K (working on W2K) with a few other certs, too...