Array In Forms

i have an array in an HTML form.

UserName[1], UserName[2]...
all the UserName r actually a checkbox.
now i need to disable them upon a disable button.
in my function i created a loop
for (Counter=1; Counter <=CheckMarkCounter; Counter++)
//CheckMarkCounter is alredy set to something like 5.
{
document.Form1.UserName[Counter].disabled=true;
}

but that failed to work.
so i tried
to do the following, write the statment into a string and run it by using eval:

st='document.Form1.UserName['+Counter+'].disabled=true'
eval(st);

as for the string, its fine but still the eval gives me an error: document.Form1.Username.1 is null or not an object....