Tricky Autonumber One.

im creating an error reporting database.
for this the hardware/system items need entering
they will have a unique id
to assist identification i would like the unique id to start with a 3 letter code refering to the type of hardware/system/periferal followed by the number
e.g a printer will ahve the id "ptr 00000" and a computer will have the id "cpu 00000" with the 3 letters automaticaly comeing from a combobox field in the same table (form to the user)
i have allready been able to get autonumbers with 3 letters before when i created the tecnitian id they are "TEC-00000"

any help thanked. im an A2 student so i know all the code stuff.

thanks in advance

Jimmythemadman.