As Raghu explains - check out the ascii table.

The 8051 has no problem with ascii chars....

mov a,SBUF
cjne a,#':',not_colon ;most assemblers accept this


Lets say we have a table of 8 bytes, each with the value from the a/d and we want to send them out the 8051 serial port to the PC:

atod_values: ds 8 ;in internal ram

mov a,#':' ;start token
call pchar ;sends A out the serial port
mov r5,#0 ;clear checksum
mov r0,#atod_values ;->data to send
mov r2,#8 ;loop count - 8 bytes of data
loop:
mov a,@r0 ;get data
push a
add a,r5
mov r5,a ;accumulate the checksum
pop a
call phex ;convert to ascii hex & send
inc r0 ;->++
djnz r2,loop
mov a,r5
com a ;negate the checksum
call phex
mov a,#13 ;add carriage return
call pchar
mov a,#10 ;add line feed
call pchar
; end of send code...

;
; converts value in A to ascii hex and sends
;
phex:
push a
swap a ;get hi nibble
call tohex
pop a
call tohex
ret

tohex:
anl a,#0fh ;low 4 bits only
add a,#'0' ;add ascii offset for 0...9
cjne a,#'9'+1,tohex_1
tohex_1
jc tohex_2
add a,#7 ;for A..F
tohex_2
call pchar
ret

pchar:
jnb TI,pchar
mov SBUF,a
clr TI
ret

See, I've done half the work for you! The visual basic side you can do for yourself - besides I haven't used VB for some time! You're in the right area when talking about ON_COMM events. Use 'lineinput' to grab the text line (VB looks for the CR or LF to figure out the end of the message). Use the VB string features to pull apart the receive string and convert back to data to process and/or display. Job done. First, get the 8051 code going and use Hyperterm to check that the data is being sent correctly - then use VB. You can choose whether or not to check the checksum value - get the basics going then add the checksum calc to your VB code to check that the data was received correctly.

p.s I wrote this code off the top of my head - don't expect it to be 100% although it should be close!