session problem

I want to identify whether user has logged in, in order to give them different authority. I know how to use queryString, but I don't like it, as some parameter will display on the URL bar.... I tried to use sessions. I defined Session("Username") in the login.asp file and use the following code to check whether it has got parameter. If it is true it means user has logged in already.  

If IsNull(Session("Username")) = False then

However, the result is always to be true, even the username actually dosen't have a value. Anyboy please give me some help to solve this problem. thanks

Thanks MorningZ! It is works!

Where would I be able to find out the detailed reference of asp, such as "Trim" you used

i typically avoid MSDN's site like the plague... but its a good resource

better ones IMHO would be devguru (VBScript link, their javascript section is also excellent) and w3schools (learn ASP)... also make sure to check out the "Favortie ASP Help Pages" thread that i stickied at the top of this very ASP forum