Hi Igor,
Some viruses can inject themselves into other files that exist on disk, so if your machine got infected it could embed it into existing files. What I would do is download mojoportal from codeplex and see if you get the same result scanning the mojoPortal.Web.dll in the download. I would also compare the file size and modified date vs the copy that kapersky reports as infected. In the end it might be a good idea to send the file to them for analysis.
Best,
Joe