I know that there are other threads out there but I found them painful to troll through. I've just 'fixed' (it's not my ideal solution) the problem I had with BOINC working perfectly for all my other projects except WCG.

Symptom:-
I could join the project but the dowload would fail with log entries as follows

09/09/2007 09:50:05|World Community Grid|Sending scheduler request: Project initialization
09/09/2007 09:50:05|World Community Grid|Requesting 1 seconds of new work
09/09/2007 09:50:07||Project communication failed: attempting access to reference site
09/09/2007 09:50:08||Access to reference site succeeded - project servers may be temporarily down.
09/09/2007 09:50:10|World Community Grid|Scheduler request failed: peer certificate cannot be authenticated with known CA certificates
09/09/2007 09:50:10|World Community Grid|Deferring communication for 1 min 0 sec

Since my Linux machine was merrily crunching, I knew the servers weren't down so looked further.

Solution:-
In Kaspersky's settings (Green Tick at the top of the window), under Service > Network Settings, I have it checked to "Check all encrypted connections. If I turned this to "Do not check encrypted connections, BOINC began the downloads, turning it back to "Check all..." stopped the download process. Since this is a security breach for my Vista PC I then clicked on the "Protection" option (on the left pane at the top of the tree). Under this option is a button for "Trusted zone...", and in there is a tab for Trusted Applications. Hit the add button and add boinc.exe and boincmgr.exe. for each, edit and click the "Do not scan network traffic" property.

"Ok" the windows, "Applying" as necessary. This solved the problem (although I'm not stunned at having to 'trust' two apps for network traffic).

Hope this helps others

Cheers

Pete