Our site is http://www.oaklandsymphonychorus.org, running DNN 5.6.3.  Today I got email from Google saying we have been infected with malware.  Here's the Google message:

Of the 7 pages we tested on the site over the past 90 days, 3 page(s) resulted in malicious software being downloaded and installed without user consent. The last time Google visited this site was on 2012-06-29, and the last time suspicious content was found on this site was on 2012-06-29.

Malicious software includes 3 trojan(s).

Malicious software is hosted on 1 domain(s), including iekcdnorebgvknkc.ru/.

This site was hosted on 1 network(s) including AS32475 (SINGLEHOP).

I've briefly reviewed the site and am deeply suspicious of the /httpdocs/Default.aspx, which was last updated at 11:57 PM 6/28/12. I didn't update it, and nothing else in that directory has been touched since April 25, 2012.  I have a site backup from 6/11 from which I can get a clean Default.aspx, but I don't know if that's all I need to do.  A Google search on the .ru site produced nothing. 

Google Webmaster says it sees 3 problems, two in / and one in

/OurMusic/CommunityEvents/SummerSingIns/tabid/171/Default.aspx

but clicking on alal 3 errors produces "Google could not locate the malicious code." 

I'm out of my depth here.  Google searches produce tools that are supposed to clean trojans off your web site, but which ones can I trust?  Any advice VERY gratefully received.