If you know what fields will be represented then it's not too difficult. Create an HTML page with the requisite form and text inputs representing your query string, e.g. if your querystring was forename=joe&surname=fawcett then have two textboxes named forename and surname.
Read the querystring using the search property and create an array using the split method with & as the separator. For each item in the array split agin on the = character and check the value of the first element, this will be forename or surname in the example. Assign the second element of the array to the appropriate textbox. Then call document.forms.submit() to submit the form. You can debug all this by having a confirm() in between assigning the values and submitting to see if everything is okay.
A completely different approach would be to use the XMLHttpRequest. Capture the querystring as before. Open a connection to the new URL with the POST method and send the entire querystring as the data argument to the send method. That way, as long as there's a one-to-one mapping between the originally querystring and the new parameters you don't need to parse the querystring at all.
Joe (Microsoft MVP - XML