In v5.5.23 on XP SP3, the server variable system_time _zone is set to a wrong value on server startup. Its value is "ope" (without the quotes), not a valid time zone designation.
IMHO, this is possibly derived form the string "Westeuropäische Sommerzeit" which is returned by the GetTimeZoneInformation() windows API call on my machine (meaning CEST, +02:00).
Note: I have an environment variable called TZ, whose current value is "CET" (without the quotes).
If I unset this variable inside a cmd.exe window, and then start mysqld.exe --console from same window, system_time _zone will be different, namely "Westeuropische Sommerzeit" (without the quotes) - note the missing "ä".
Again, this appears not to be a valid timezone name according to the mysql.time_zone_name table.
The correct value would probably be CET or Europe/Berlin.