De-Fraggle rock

Today I have mainly been trying to defrag the compluter. I understand the principle of it okay, and know how to get the thing to start doing it. But it’ll get to about 10% and then inform me that the drive’s changed and it’s restarting. I asked several people what I should do and was advised to make sure that no other programs are running at the time as they’ll be causing the problem. Righto – how do I do that? On desktop, hit Ctrl/Alt/Del I was told, then shut down the programs on the list one at a time.

I’ve found a snag with that. If I click End Task at the bottom of the box, the highlighted program certainly stops running, but the list box also goes away. If I Ctrl/Alt/Del in an attempt to get it back so I can stop more programs, the computer shuts down. No matter what I do I can only stop one program – not all of them (I was told to leave Explorer running to keep the computer going). Last night I had to restart the computer 4 times as I worked my way through all the available options. I can’t even manage to highlight all the programs I want to stop at once. Usually holding Ctrl while clicking down a list selects more than one, but that doesn’t work with this.

So I thought I’d google for instructions on defragging. D’oh! It’s not really helpful to simply be told that ‘It’s best to shut down all other programs while defragging’ without being told how. So I kept on trying and eventually succeeded on getting it so that only Explorer was showing as running - screensaver shut down and everything. Hurrah! Here we go!

Nope. Just the same. Gets to 10% then says the drive's changed. I’m stumped. Suggestions? Or do I learn to love the effect of wading through treacle?

Edit: Gordon, you're a star! F8ing at Startup and going into Safe Mode did the trick. Thank you!