Google Internet Trace Destructor. The last such software I had was by a company called Steganos. It came with a secure delete function. Nothing is perfect, but only the expertise of the NSA or GCHQ should stand any chance of recovering files.

As for your personal files, well you've two options:

- You can move them to Skydrive, Google Drive or Dropbox (if you're prepared to trust them)
- Or you can keep them on backed up external drives.

I personally favour the Google 'everything online' way of doing things, but remain uncertain as to whether or not Google can be trusted with personal data and IP.