Ctrl-W is a common shortcut key that many programs use for "close current window". Many programs use this, including:
- Office programs such as Excel, Word, and PowerPoint
- Web browsers such as Internet Explorer, Firefox, and Chrome
- Document programs such as Adobe PhotoShop and more
While they all share this same shortcut, each one implements it independently: Windows itself has no way to shut off this shortcut universally. Therefore, your options are limited. There might be 3rd party software that can reserve the key, but that is very doubtful because Ctrl is so oft-used. Sorry for this unfortunate news.