You might try the beta drivers from Diamonds web site ( www.diamondmm.com), but I seem to remember someone saying they had problems with them ...? Do a search of this site to check that
 
I used the Lucent56k USB drivers found under the Compaq section for add modems. This solution worked great for me on a number of USB modems that use the Lucent chipset (CNet USB, Supra USB, etc), until I installed the Hotfix for Win2k. Now it doesn't show up in the list... but my modem that was installed prior to putting the fix on, still works fine.

Using the S3/Diamond beta drivers are a bad idea. First off they don't work. The installation won't finish because it can't find a file and then terminates. It never reported what file it was looking for. After that, you can't install any other USB modem driver, because it keeps defaulting to that useless Diamond driver which won't install.
 
Go to this site: http://www.zoom.com/techsprt/win2k.shtml
 
and download the Zusb2000.exe file. Extract it to a dir of your choice. Plug in your Supra modem, and when it asks you for a driver, select the .inf file from Zoom for your modem and it should work no problem. It did for me anyway. I don't have 56k v.90 lines up here, so I can't guarantee that will work, but 33.6 worked excellent with no random disconnects. Gaming was also very steady in positional games like NFS Porsche and Motocross Madness 2.
 
Hope this helps.