Have you tried looking for a product code on the modem. Looking for any codes on the modem and then try entering them into Google. Take a look at the search results and it should give you an indication of the make/model etc. Then try going to the manuafacturers website to download the drivers. Don't bother going to website slike Mr Driver etc because they will only try to take your hard earned for something that should already be free.
P.S the above will obviously only work if you have access to another PC that is already on the net!