Puppy Linux is pnp with USR 5637 modem
I was browsing around quite a bit in Linuxquestions and on Ubuntu's doco trying to get my USR 5637 modem to work with Ubuntu 9.10 on my Dell Optiplex GX240. I then saw a post somewhere that said they struggled a lot with Ubuntu, then went to Puppy Linux and got their US Robotics 5637 to work real easily. I also saw a post on Linuxquestions saying "Dell + Ubuntu + modem = disaster"
I downloaded Puppy Linux 4.2.1 and found it to be totally plug and play with the US Robotics 5637 modem.