Login page code enhancement
I have the username and password for the admin stored in Web.config (Is that a good practice?).


No, it's not good practice to store the Admin login (or any login, for that matter) in the Web.config.

Depending on what ASP.NET model you're using, you have a few options. MVC has a built-in Roles provider which makes it simple to make certain pages/areas only accessible by  a specific role.

Web Forms is also pretty simple, though you have to roll your own access provisions. You can use the built-in Simple Membership if you like, which usually suits quite well once extended to fit your needs. (You can add properties to users pretty easily and such.)

You also aren't encrypting or hashing the password, which is a huge no-no. At the very least you should MD5 hash it, though for security I would recommend SHA1-salt hashing at the very weakest. Though, if you use the built-in Simple Membership for Web Forms or MVC, you'll get encryption and salting by default.

I didn't review the actual code as Heslacher did that just fine, but I do think you need to completely revisit the login system. ASP.NET has built-in providers for this, and they're super easy and, quite honestly, pretty fun to set up. (I don't know why no one has pointed this out in the several months this question has been here, or how it got on my radar, but hopefully you still get this advice.)