There are tools that will permenately erase your hard drive where data will NOT be recoverable. Google for "killdisk" - not sure if there is a MAC version, but you might find something like it if it doesn't support the MAC.

As for cookies and spyware, you should always run Tracks Erase Pro and Privacy Eraser to clear out cookies and your tracks, and use Mozilla (not Internet Explorer) to surf the web - MUCH more secure. Google for those.

These can all be configured to automatically clean your system of ALL the garbage that gets put on the hard drive when you shut down your system. I use these religiously!

And while you're at it, Webroot's Spy sweeper also cleans out all scumware (spyware, key loggers, etc.). Oh, and use ZoneAlarm (free) for your firewall, and use a router with a built in NAT firewall. And don't forgot an anti-virus tool (Norton). You can get ZoneAlarm by going to their website and downloading the free version. Works GREAT!

The internet is a dangerous place and not very secure...you need to protect yourself and your system at all times.