This program has performed an illegal operation and will be shut down??

Hi, I'm buiding an application to get data from a device. this device using auto comm mode (send data everytime). I could open the port, and get the data properly good. but when i closed the port(through close icon on right top form), sometimes but almost appear this error "an error occured that said the progam has performed an illegal operation and will be shut down".
but if i closed through command button (just unload me code inside it) it never get the same error.
I'm using an activeX Exe program to communicate with device.
I create an instance to the activeX Exe and kill it when unload form.

this below is the detail of error cause :
DRGABCOMM caused an invalid page fault in
module KERNEL32.DLL at 017f:bff7b9a6.
Registers:
EAX=00000000 CS=017f EIP=bff7b9a6 EFLGS=00000246
EBX=004164d0 SS=0187 ESP=0063fb60 EBP=0063fb7c
ECX=0041624c DS=0187 ESI=004164d0 FS=5d57
EDX=0107003c ES=0187 EDI=00000000 GS=0000
Bytes at CS:EIP:
ff 76 04 e8 13 89 ff ff 5e c2 04 00 56 8b 74 24
Stack dump:
0041624c 21c193a3 004164d0 00000000 0041624c 00000001 0041624c 0063fba0 21c1963c 0063fb94 0063fbb8 00008c28 0063fc04 00000000 0000002d 00000000

please help me to solve this prob..
thank you


Liung 012