[wbillingsley]

Word Perfect 5.1 for DOS, released in 1989, was very popular indeed among Microsoft's customer base. And the first to have a (DOS) menu bar.

Open the File menu and 'x' is highlighted in red in the word 'Exit'.

In other words, prior to Windows 95's release, DOS programs had already set firmly in Microsoft's customers' minds that 'x' was for exit.

RISC OS (1987), used on Acorn computers in schools in the UK also used an 'X' close button on the windows. And look at what word is prominent in the bottom right of the desktop... exit

http://www.dasmirnov.net/media/blogs/blog/bigarthur.gif

It seems that as soon as the word "exit" is prominent in your UI, 'x' is going to become associated with exit.

An enormous amount of Microsoft's target market were already using Word Perfect every day and seeing red 'x's for exit. So when they decided to put a button on the window bar, what else were they likely to pick?