hey waseem,
this sql server is not installing the instance...because there is already Instance installed on your computer
there are two ways to solve this problem...
1. you didnt uninstalled everything related to sql in "uninstall programm".. so first revoce everything related to sql in uninstall program then restart you system.... then install new sql server....
2. if you still getting this problem.. after doing step 1 .... then when the instance screen come.....there is option whether to go with default instance or Named instance then click on Named instance and name any instance.... simple...
so this way you can sort you problem.......