Sergey,
This issue is not something we have come across before.. Could you please clarify whether the problem occurs when Cassini runtime components (Server and Explorer) are not installed? If the issue is in checking or trying to load prerequisites, then by installing Explorer and Server components from stand-alone MSIs you will make Cassini for VS2005 skip runtime components download and installation step. Please try installing Explorer and Server first before installing Visual Studio integration components.
Please let us know if this has helped. If not, we'll try to come up with the way to get Cassini into visual studio prereqs list some other way.
Best regards,
UltiDev Team.
Please donate at
http://www.ultidev.com/products/Donate.aspx to help us improve our products.