You could use word. Just add the image to word and resize it to fit into the printable region (Use one corner to resize, as resizing it another way would stretch the picture). Right click the image, then select "Format Graphic" -> "Layout" -> "Behind Text". Afterwards you could put text above the image. If you want you could use Text Boxes which would be faster in your Situation i guess. Doing this is not the best solution (This will probably look different on different Word versions, you will probably not be able to open it OpenOffice without significant changes (And it does not feel right for most computer enthusiasts)), but it is fast and should be sufficient for a tax form. And as you probably know how to use word you won't have to install and learn how to use another software.
Depending on what you want to do, you could fill the text boxes with a white background (You could use this to wipe out regions of the tiff, or use a transparent background to make it look more native to the .tiff.
1Caution: This form of editing leaves the original image data available to anyone viewing the word document. If the changes are sensitive be sure to export the results into a format that does not include the original image. – Chris Nava – 2011-03-14T04:44:28.147
@Chris Nava Good Point. In his case it should be no problem though. – Darokthar – 2011-03-14T07:57:14.193