Please find list of Canon error codes
http://www.camerahacker.com/Canon/D60_Error_Codes.shtml
From the same site...
Err 99: SYSTEM
DESCRIPTION: This error occurs if there is an internal malfunction detected during the camera's self-checking procedure that's executed before every attempted exposure. This is somewhat similar to the flashing 'BC' warning that occasionally appears on EOS-1 series 35mm SLRs.
REMEDIAL MEASURE: Try releasing the error by turning the main power switch OFF and then back ON. If this does not solve the problem, have the EOS D60 examined by a qualified service technician. Please be careful to note the conditions under which the error message occurred. This error is similar to Error 09 or Error 10 on the D30.
Hope this helps