(This page is only intended for Internet Explorer.)
Occasionally, Internet Explorer may stop using AlternaTIFF to view TIFF files, even though AlternaTIFF is still installed. This web page instructs AlternaTIFF to attempt to reconfigure IE to use AlternaTIFF to handle TIFF files.
Unfortunately, this "reactivation" feature will usually not work in Windows Vista. Workarounds include uninstalling and reinstalling, or temporarily running IE as an administrator and returning to this page.
You should be seeing the question "Do you want to reconfigure Internet Explorer to use AlternaTIFF to view TIFF files?" Answer Yes to try to make AlternaTIFF the default TIFF viewer again. Then visit the test page to see if it helped.
If nothing is happening, then you may have a more serious problem.