There are only two drivers listed for win 2000 and the 2400, so if you tried them both and neither work, you may be out of luck with the existing modem. In Device Manager, are there any yellow exclamation marks on any of the items? Does your modem work at all?  Also, have you reinstalled windows or made changes that necessitate a new driver installation?

Following a Windows reinstall, you should, at the very least, install this chipset driver, which you may already have done:

http://support.dell.com/support/downloads/download.aspx?c=us&cs=19&l=en&s=dhs&releaseid=R47822&SystemID=DIM_PNT_P4_CEL_2400&os=WNT5&osl=en&deviceid=3268&devlib=0&typecnt=1&vercnt=1&formatcnt=1&libid=27&fileid=56457

There are also network, audio, etc. drivers which you must have done or there would be other problems. Since you're on high speed you aren't using the internal modem anyway, right? Not sure what else to tell you if everything is working well otherwise.

Try the Win2K software secton, maybe someone will know of a modem driver that will work for Win 2K.  In many cases Dell only provides drivers for the Operating System that the PC came with.