All of a sudden, Ctrl+Shift+A stopped working in Outlook and Windows Mail. In Outlook, I used it to create a new appointment, and I use it in Windows Mail to mark all messages in the displayed newsgroup as Read.
Long story short: It was Trillian, which I recently started using, albeit sporadically. Until yesterday, apparently, I had not tried using Ctrl+Shift+A while Trillian was running.
Trillian was reserving, but not using, three keyboard shortcuts: Ctrl+Shift+I, Ctrl+Shift+A and Ctrl+Alt+C. Reclaiming them for your own purposes is a two step process. First, the delete them from Trillian’s Advanced Preferences. Then, you close and restart Trillian.
Right-click Trillian in the system tray, and choose Trillian Preferences. Click on Advanced Preferences. In the left pane, click on Automation. Under Automation, scroll down until you see the first reserved keyboard shortcut. Click on it, and Change… and X (delete) options appear. Click the red X, and poof! Remove any other offenders in the same way, then close the Preferences box, and close and restart Trillian. Those keyboard shortcuts will no longer be hijacked.