Assuming you have administrator rights, you need to change the server configuration. You can do this using the MySQL server tools (the workbench is the last) or directly edit the my.ini file.
Remember to enable the general log. Here is the entry for writing Errors to the log file:
general-log
log-error = "C:\Program Files\MySQL\MySQL Server 5.1\logfiles\ErrorMessages.log"
change
Oh. Here is an excerpt from my my.ini 5.0.51b file
log-queries-not-using-indexes
log-warnings
long_query_time = 2
log = "C:\Program Files\MySQL\MySQL Server 5.0\logfiles\querylog.log"
log-slow-queries = "C:\Program Files\MySQL\MySQL Server 5.0\logfiles\SlowQueryLog.log"
log-error = "C:\Program Files\MySQL\MySQL Server 5.0\logfiles\ErrorLog.log"