I just tried this (note I was using Word 6.0 but it should be the same):
I opened an ASCII text file containing some tablature. As I suspected, the default font was a TrueType or proportional font and it threw the tab off. I blocked the tab text and changed the font to Courier (NOT New Courier) which I know to be a non-proportional font and it all straightened out. This is because non-proportional fonts use the same amount of space for all characters, whereas proportional or TrueType fonts use less space for skinny letters and numbers such as i and 1 and more for fat ones like M and W. So if something is all lined up like tab, written in a non-proportional font, and you change it to a proportional font, it gets all skiwampus (sp?). 'Nuff said.