Hmm, Windows XP does comes with Fax and Picture viewer that allows you to view faxes (.tif files)
I find that sometimes Windows loses the files association and cannot open tif files. To check this, Open Windows Explorer, go to Tools, and select Folder Options. Click on the File Types tab and look for .tif. Check to make sure it is there and is properly associated. If you can't find it, add it and change the default program to Windows Picture and Fax viewer.
Alternatively, if you use Office 2003/2007, you can use Document Imaging to handle Tiff files as well. Most of my clients prefer Document Imaging because you can easily access the different pages within the fax.