Modems are cheap these days. You can get a Lucent chipset Winmodem for $20USD these days.

If the modem is on a PCI card, take a look at the FCC-ID and then do a search on it.
[link|http://www.fcc.gov/oet/fccid/|http://www.fcc.gov/oet/fccid/]

It will tell you the company that made it.

See if you can figure out what the part number is for the card, and get it from the OEM that made it for Dell.

I used to have to do that for people who bought Gateway and Packard Bell systems that lost their driver disks and had the hard drive crash on them. Most Packard Bell cards were Reveal or Aztec audio cards, etc. I used to have a Geocities Reveal Windows 95 site with the drivers I collected because Reveal was out of business.