1) Press Ctrl+Alt+Del to open the Windows Task Manager. Click on the Processes tab and highlight Explorer.exe. Click "End Task" in the lower right. You will be presented with a warning, which you should select yes on.
2) If everything worked, your taskbar and desktop should have disappeared. Don't worry, it's temporary. In the Task Manager, go to the File menu and select New Task. In the resulting input box, type "logonui" and click OK. The logon screen should now appear behind the Task Manager. At this point, go ahead and take your screenshots. You can open Paint by typing "mspaint" into the New Task box.
3) Once you have finished taking and saving the screenshots, select "logonui.exe" in the process list in the Task Manager and click End Task. This will close the logon screen. Finally, go to File and New Task. Type "Explorer" into the text box and click OK. Your taskbar and desktop should now re-appear.
That's it! Hope this helps.