Someone is trying to hack me

Hi, I have installed wordpress 3 days ago and since the installation, there is someone or a bot keep trying to hack my wordpress site.Here is a little info on the person or bot trying to hack me.

188.138.56.125
2012-09-11 04:40:58
[404] /e6fd5a264f812fbbf22aa86911e7dcced8c0f36f.php
Referrer: Direct hit
Hostname: cls2s1.websitedefender.com
Probably hack attempt!
04:40:57 ->[404] /e6fd5a264f812fbbf22aa86911e7dcced8c0f36f.php
04:40:58 ->/

188.138.56.125
2012-09-11 03:25:54
[404] /e6fd5a264f812fbbf22aa86911e7dcced8c0f36f.php
Referrer: Direct hit
Hostname: cls2s1.websitedefender.com
Probably hack attempt!
03:25:53 ->[404] /e6fd5a264f812fbbf22aa86911e7dcced8c0f36f.php
03:25:54 ->/

188.138.56.125
2012-09-11 02:10:50
[404] /e6fd5a264f812fbbf22aa86911e7dcced8c0f36f.php
Referrer: Direct hit
Hostname: cls2s1.websitedefender.com
Probably hack attempt!
02:10:49 ->[404] /e6fd5a264f812fbbf22aa86911e7dcced8c0f36f.php
02:10:50 ->/

188.138.56.125
2012-09-11 00:55:46
[404] /e6fd5a264f812fbbf22aa86911e7dcced8c0f36f.php
Referrer: Direct hit
Hostname: cls2s1.websitedefender.com
Probably hack attempt!
00:55:43 ->[404] /e6fd5a264f812fbbf22aa86911e7dcced8c0f36f.php
00:55:46 ->/

as you can see in 3 days there have been 4 attempts.The bot or person keeps calling a strange php url on my website.What can I do to prevent this hacker or hack attempt.