I use W2K for OS. IE5.0 with OS install added AR4.x to system and could use browser integration to view .PDF files and .TIF files. Worked great.

Only change is upgrade to IE5.5 now only thing that will veiw in integrated browser is .PDF. The .TIF files that used to work great now it wants to download the file and open in AR without browser integration but errors on file does not start with %PDF which makes sense since it is not a .PDF.