Dear Nguyen:

Your problem could be come form different sides.

1- Do you have a optocoupler connection between MCU and PC serial port ??? ...if yes, check the correct optocoupler for that purpose, ie: high speeed opto, like 6N137.

2-What baudrate speed are you using ?....try to use low baud rate speed's like 9600 bps (if it's possible), and this will minimize your problems, high baud rate speeds, increase the possibility to lose data if you have not a correct design, of your interface and cable characteristics.

3-Be carrefully, think that your VB software is very very quick that your MCU, insert correct delays in the software.

4-I reccomend that you use HANDSHAKING, so you can controll the exactly moment when the data transfer occur.

5-Using a first "check" byte or bytes, could be a good idea too, that lets you to controll if the correct string is comming in from MCU. ie: "D,A,T,A,BYTE1,BYTE2,BYTE3,BYTE4"

6-Another good idea is : to empty the recieve buffer after recieving the full packed data in the VB software.

To finish, your problem could be a "mix" of my mencioned points, try to implement at least, Handshaking, 9600 bps, check the cable connection and quality, and interface design.

One more....are you using MAX232 or similar level converter ??
or a simple R-transistor method ?