The choice of a lightweight distribution is not enough to ensure that a system can comfortably be run on an old computer. It is equally important to select lightweight applications that do not demand too much from the limited hardware. For example, OpenOffice.org is a great office suite but it can be too heavy for the old boxes with low memory.
I do most of my word processing with other applications. Sometimes I just use Emacs for simple text documents or LyX if I want to have a nice and well balanced printout that is typeset with LaTeX. But usually when I have to write a paper for the school I use Abiword.
Abiword does a reasonable job in importing from and exporting to RTF and DOC. It is not perfect but usually good enough for my simple text documents that don't have any graphics. If I need to edit documents that include graphics I usually edit the documents with OpenOffice and finish them using MS Word at school where I can ensure that the document is formatted using the official template of the school.
I like also the good selection of import and export plugins available for Abiword. There are plugins for all kinds of applications ranging from nroff and LaTeX to WordPerfect. The plugins are not perfect but usually good enough.
Abiword has been localized in many languages. Try Abiword, it might well be good enough for you.