In this strange complex of time and space anything can happen

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?

This is one for my computery friends:

I use an AVG virus checker, which I update once a week and which runs automatically once a day. I've been used to it taking about 20 minutes to run its daily test (of the whole C drive), or more if I am using my computer at the same time.

However, for the last couple of days it has suddenly been taking only about 10 minutes to run the same test. Nothing significant has changed about my C drive, so why might this be? And should I be worried: could it means there are parts of my C drive which now aren't being checked for some reason?

Everything else seems to be in full health, by the way.

Hope someone can help!