You can store your interval reference in a variable, then when you hover the image stop the interval and when you exit start it.
Code:
var theInterval;
function startSlide() {
theInterval = setInterval(slideSwitch, 5000);
}
function stopSlide() {
clearInterval(theInterval);
}
$(function () {
startSlide();
$('#slideshow DIV').hover(function () {
stopSlide();
}, function () {
startSlide();
})
});