Ajax :: Stops Working In IE8 After Reload?

I have a simple function for an intranet site that should make and return a XMLHttpRequest object in most browsers (including IE8):

function createXMLHttpRequest() {
var xmlhttp = false;
if (window.XMLHttpRequest) {
xmlhttp = new XMLHttpRequest();

[Code]....

Then I use it autocomplete a input text with a list of client company names from a database. Then the user selects a company and it reloads the page with that company's info. (The ajax input text box remains so the user can type in another company name)

In firefox this works fine. However, in IE8 the very first time the page loads it works fine, but after the user selects a company and the page reloads, the ajax longer works. It gives an error at "xmlhttp = new XMLHttpRequest();" saying: "Message: Object doesn't support this property or method"

If I type anything in the input, it gives the error: "Message: 'searchReq.readyState' is null or not an object" leading me to believe that somehow after the first time the page loads, IE is not able to process a XMLHttpRequest which seems unlikely, so I'm not sure where to go with my code. I've tried a bunch of other similar createXMLHttpRequest() functions, but they all have the same problem of working once then not working after.