Anytime you copy data on your computer, that data is stored into your computers memory called the clipboard.

This can be a huge security issue.

For example, lets say you want to login to a website and are tired of typing in the password over and over so you copy and paste it in the password field. Your password is being stored in your computers ram (memory) and it wont be released until you either logoff or reboot your machine.

Microsoft office and probably many other programs can store multiple copied or cut data items into memory to restore at a later time.

How to clear the clipboard.

First you'll want to make sure there is data in the clipboard.

Open a document, copy some data into memory.

Paste it somewhere in a new or existing document. 

Open the windows command prompt as administrator

Type clip /? to see help on the clipboard.

Type "echo off | clip"

Press the enter key.

Open a blank document and press the Ctrl + v key.

No data should have been pasted. Data in memory (the clipboard) should have been cleared.

However, Microsoft still retains this data in several files which I have still to find.

Search windows for Clip.exe