Below are IIS7 logs I generated after attempting to login twice (once locally at server, once remotely). Still getting the 501 error.
In this case, the /installer folder is not present on the site.
$rcmail_config['enable_installer'] = false in main.inc.php config file
System: Windows Server 2008, IIS 7, PHP v5.2.11 via FastCGI
Hopefully, this is what you're looking for, although it doesn't show any errors. I think what texas12 said is correct: the screen that shows "501 Error" is generated within the PHP code logic based on program conditions - the "501" itself doesn't appear to generated by IIS.
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#Software: Microsoft Internet Information Services 7.0
#Version: 1.0
#Date: 2010-07-27 01:06:33
#Fields: date time s-sitename s-ip cs-method cs-uri-stem cs-uri-query s-port cs-username c-ip cs(User-Agent) sc-status sc-substatus sc-win32-status sc-bytes cs-bytes time-taken
2010-07-27 01:06:33 W3SVC17 174.121.41.203 GET / - 80 - 174.121.41.203 Mozilla/5.0+(Windows;+U;+Windows+NT+6.0;+en-US;+rv:1.9.2.3)+Gecko/20100405+Namoroka/3.6.3 200 0 0 944 471 2028
2010-07-27 01:06:37 W3SVC17 174.121.41.203 GET /favicon.ico - 80 - 174.121.41.203 Mozilla/5.0+(Windows;+U;+Windows+NT+6.0;+en-US;+rv:1.9.2.3)+Gecko/20100405+Namoroka/3.6.3 404 0 2 5461 533 0
2010-07-27 01:08:18 W3SVC17 174.121.41.203 GET / - 80 - 70.105.143.44 Mozilla/4.0+(compatible;+MSIE+8.0;+Windows+NT+5.1;+Trident/4.0;+Media+Center+PC+3.0;+.NET+CLR+1.0.3705;+.NET+CLR+1.1.4322;+.NET+CLR+2.0.50727;+.NET+CLR+3.0.4506.2152;+.NET+CLR+3.5.30729) 200 0 0 873 746 436
2010-07-27 01:08:18 W3SVC17 174.121.41.203 GET /skins/default/common.css - 80 - 70.105.143.44 Mozilla/4.0+(compatible;+MSIE+8.0;+Windows+NT+5.1;+Trident/4.0;+Media+Center+PC+3.0;+.NET+CLR+1.0.3705;+.NET+CLR+1.1.4322;+.NET+CLR+2.0.50727;+.NET+CLR+3.0.4506.2152;+.NET+CLR+3.5.30729) 200 0 0 9662 562 327
2010-07-27 01:08:18 W3SVC17 174.121.41.203 GET /favicon.ico - 80 - 70.105.143.44 Mozilla/4.0+(compatible;+MSIE+8.0;+Windows+NT+5.1;+Trident/4.0;+Media+Center+PC+3.0;+.NET+CLR+1.0.3705;+.NET+CLR+1.1.4322;+.NET+CLR+2.0.50727;+.NET+CLR+3.0.4506.2152;+.NET+CLR+3.5.30729) 404 0 2 1405 469 93
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Contents of /logs/errors file:
[26-Jul-2010 21:06:33 -0400]: PHP Error: Error loading template for login in C:\HostingSpaces\Internal\roundcube.mail.somedomain.arvixevps.com\wwwroot\program\include\rcube_template.php on line 375 (GET /)
[26-Jul-2010 21:08:18 -0400]: PHP Error: Error loading template for login in C:\HostingSpaces\Internal\roundcube.mail.somedomain.arvixevps.com\wwwroot\program\include\rcube_template.php on line 375 (GET /)