Internet 5.x and 6.x (and probably all earlier versions) have a bug that prevent proper viewing of tab-separated exported data when non-default Text Size settings are used with these browsers. Specifically, exporting the Traffic -> Daily report in tab-separated format when the Text Size setting is Smaller or Smallest will result in the Date and Visits columns being merged together in the output. Other reports may be affected as well.
There is no fix since this is a bug in the way that Internet Explorer displays data with a PRE tag. The workaround is to use a Text Size of Medium (the default) or larger.