It is possible but only in Netscape Navigator 4 and Internet Explorer 4. You'll need to use a link - but you can make the link without the underline and without it going anywhere.
I've used an alert, but this can be replace with code that does whatever you want.
The following was submitted by Peter A. List
Easy in Internet Explorer 4+ - just include the actions & events inside a div or span tag.
<form name="Out"> <input type="text" size="35" name="put" onFocus="blur()" value="You have not yet moved over the text"> </form> <span onMouseOver="document.Out.put.value='Now you are over the text'" onMouseOut="document.Out.put.value='Hey, you moved away from the text!'"> Mouse over me, and the text in the input will change </span>