State Management :: Session Variable And Multiple Web Application Instance

I have a web application using window authentication. There are a search page and a edit page in the application. In search page, user can enter some search criterias in the textboxes and the search result will be displayed in the gridview by clicking the "Search" button. User can select any row from the gridview and it will take him to the edit page. In the edit page, there is a "Back" button to take the user back to the search page. I use session variables to remember what the search criterias the user entered in the textboxes and what page number the user was in the gridview. So when the user goes back to the search page, the textboxes are pre-populated with the search criterias and the gridview is displayed in the correct page number.

The problem is: If user1 opened one browser window and did a search using search criterias1, then user1 opened a 2nd browser window and did a search using search criterias2. When the user1 went back to the search page from the edit page using the first browser window, all the textboxes and page numbers are from the 2nd browser window, instead of the 1st one. Can anyone explain this problem to me and how to prevent it? It will be helpful too if you can give me some links about the similar problem. I am thinking if the same user logged on two different machines and did the search on each machine, what is the result?