Someone is running a scanner on your machine for known exploits. If you get a stealth scanner such as this one http://www.nstalker.com/stealth.php
and run it against your site you will know then if you have problems or not. There are freeware scanners by the dozen so finding one shouldn't be a problem. Also get the Microsoft IIS Server hardening Guide and go through it. It does need to be done. The Microsoft site has this guide. Looking at your log sample there doesn't seem to have been any success with the scan. The 3 digit number after the command information is the return code. See this site for there meanings http://www.webopedia.com/quick_ref/error.html There should be fuller information on other sites or your web server help files for these return codes. You can use www.samspade.org to find the culprit in question. If they are not spoofing their address. DO NOT try to connect to them direct from your site, even with a ping. Best of luck Trevor Cushen
