Why does Windows sometimes "copy to clipboard" when I press the delete key? [duplicate]

Solutions to this can either be found in this question:
How do I remap Shift + Del on Windows to mean Delete instead of Cut? or in Jeremy's own answer.

You're most likely pressing Shift+Del.

You still have Shift pressed from making your selection. Shift+Del is the key combination to Cut (just like Ctrl+X).

* SOLUTION *

After Oliver pointed out that shift+delete is a keyboard shortcut for Cut (just like Ctrl+x) I started looking for a way to turn this "feature" off.

The only solution I've found is to use AutoHotKey (free). Here's some basic setup instructions for anyone else that's interested:

First, install AutoHotKey, run it and allow it to create (and open) the example/default AutoHotkey.ahk script when it prompts you. Add the following to the script:

+Delete::
    KeyWait, Shift
    Send, {Delete}
    return

I'm not sure but you may have to "reload" the script before it starts working (I reloaded before testing). To reload, bring the AutoHotKey app to the foreground and press Ctrl+R (or click File -> Reload Script).

If you're happy with the results then you'll probably want to setup AutoHotKey to run automatically on startup. This is done by selecting the AutoHotkey.ahk file (in your 'My Documents' folder) and then going to your startup folder (C:\Users\%username%\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup) and clicking "Paste Shortcut" from the "Edit" menu (Press Alt+E if you don't see an menu bar).

In testing, this fix worked great for me on Windows 7. Thanks to Oliver for identifying the underlying problem! :-)

Another possibility is a Sticky Key accessibility feature in Windows 7, so you pressed Shift key before and when you press Del, Sticky Key combines them to Shift+Del, which is another shortcut for Cut (to the buffer).

I found following article helpful in figuring out the setting - Stuck shift keys - windows 7 64 bit.