make a global variable
int time = 0;
then your code can be...
private void Timer_Tick(object sender, EventArgs e)
{
switch (time%5)
{
case 0:
if (txt1.Text != string.Empty)
SendKeys.Send(this.txt1.Text);
break;
case 1:
if (txt2.Text != string.Empty)
SendKeys.Send(this.txt2.Text);
break;
//finish the switch
}
SendKeys.Send("{ENTER}");
time++;
}
}
you could even use
this.Controls.Find("txt"+(time%5 + 1))