This is a false positive, undoubtedly introduced with a recent virus definitions update. Apparently the installer has some file or operation properties that make it look like graybird. I've seen several tests/reviews that reported Norton Internet Security to detect an exceptionally large amount of false positives, so in that light this may not be a special case.
Please report the problem to Symantec, with a link to the file that is reported as infected. They will investigate the file, and when they find that it is indeed a false positive they will update their virus definitions accordingly.