VS 2008 Timer Delay In VS 2008?

I am using serial port to send packets in a form. There is a delay between two common packets, say 20 seconds. I used to use sleep to delay 20s before the second packet send.But sometimes I need to send a special packet every 4 seconds to keep device alive before the 2nd packet send. But during sleeping I cannot do anything.Per a suggestion yesterday from an expert, I changed to use timer solution. I set Timer1.interval = 4,000 and Timer2.interval = 20,000 and enable Timer1 and Timer2 after the 1st packet in Form1. After the 2nd packet is read to buffer, I use "while" to wait there until 20s overflow in Timer2 to send this common packet out. At the meantime, Timer1 is supposed to overflow every 4s to send the special packet. Now the problem is: