How to overcome the problem associated with querystring ?

Hi ..

i have developed a website. As usual im using quertystring to passs value from one page to another .

EX:If im moving from page ABC.aspx to PQR.aspx with some querystring

Till here everythinh works fine .

my problem starts here

say i reached PQR.aspx name="Sachin"

it will show all the data related to name="Sachin"

but if somebody in querystring changes name="Nitin"

it shows data related to nitin .

i Dont want user to get details of other users .

rather if somebody changes tehn he shud be redirected to say Login.aspx

PLs provide me some tips or C# code

Looking forward for ur reply

thanks

Sachin