OK, not a prob.

Just let the SERIN2 or HSERIN do the work for you...
Code:
SerNum  VAR BYTE[5]

SERIN2 Dpin, Dmode, [HEX2 SerNum(4), HEX2 SerNum(3), HEX2 SerNum(2), _
                     HEX2 SerNum(1), HEX2 SerNum(0)]
Now the SerNum array holds the "half length serial numbers".

From there you can write it to EEPROM, test it against what's already there, or whatever.

HTH,