strange_complex | Technological conquests and a data security question (Reply)

I'm feeling pretty chuffed with myself on the technology front today. I have:

Finally got my DVD-video and Sky-box to talk to one another. It turned out that I'd got it all set up right in the first place, but just needed to choose the A1 channel on the DVD-video machine instead of letting it sit on channel 1.
Convinced my BT Home Hub to supply full wireless coverage to my entire house, rather than just a 2m pool immediately around it. Those people who said that the key to doing this was changing the channel it was broadcasting on (dakegra first, I think, confirmed by kernowgirl's husband) were really right. Setting it to channel 6 completely transformed it from basically not really working at all to working absolutely perfectly everywhere I could want it to. Amazing.

However, I still have a technological question:

Having recently bought my laptop from Dell, I want to take advantage of their partnership with ReCOM to recycle my old desktop PC to charity. I've checked that it meets their requirements, and established what I need to do to get it collected, but obviously it's crucial to ensure that it is data-safe before it goes out of my house. So far I have:

Uninstalled pretty much every piece of software I ever installed on it, with the exception of harmless ones like Adobe Acrobat
Told both IE and Firefox to clear all my personal data (passwords, browser history, favourites etc.)
Manually deleted all internet cache files, cookies etc. just to be sure
Wiped all my old documents, pictures and music (after copying them to my new machine, natch) and all temp files
Emptied the Recycle bin
Defragmented the hard drive

For the record, I never used anything other than web-based email accounts from it, so there shouldn't be old emails stored anywhere on it.

Is there anything else I should be doing before I let someone else have it? Or is rendering what was my primary personal and work computer for a total of six years truly data-safe so difficult to do properly that I'd be better off smashing the hard-drive with a hammer and taking it to the tip?