xpander, there are two obvious things to check...
1) you have already done this and it is to check smilies are enabled in your profile
2) when you are posting a message, just below the box you type in are four "tick" boxes, make sure the one that says "disable smilies in this post" is not ticked.
Then look to the left of the tick boxes for your three options, it will tell you about html, bbcode and smilies. Make sure bbcode and smilies are on.
Other than that, throw your computer away and buy a new one... a bit extreme I know, but hell it will make you feel better!