Check your software update.

The 10.5.7 Update is recommended for all users running Mac OS X Leopard and includes general operating system fixes that enhance the stability, compatibility and security of your Mac.

For detailed information on this update, please visit this website: http://support.apple.com/kb/HT3397.
For detailed information on security updates, please visit this website: http://support.apple.com/kb/HT1222.

286MB.