Unless you have a switch on your network that your connected to then setting it to 100 will possibly slow you down horribly. Let it auto detect if its possible. If your wondering why this is I can explain half-duplex/full-duplex, switching, and the basic networking principles behind it if you want. It may already be on autodetec though if your connection isn't slow and it keeps defaulting. Is there anyother information you can add to this?
Regards,
Steve

Steve, on a kind of related note, I have one box at the end of a fairly long run of cat 5 that will only work with an old non 10/100 realtec nic. If I put a 10/100 card in this box, it won't work! I've tried several 10/100 nics and none will work except this old realtec. I don't really know why this is. What do you think?

Hey Dan, hmm...sounds interesting. It'd probably be better to answer if I knew the chipsets, but regardless... Sounds like either a motherboard issue or one of the following. But first if you can, test out one of the replacement nics on a different machine, make sure it works. If the replacement nics are all the same brand/chip but they work on other machines, then its probably a motherboard issue. If its a whole bunch of different brands/chips then it is possible that the slot your putting them in is shot, unless your using the same one as the realtek nic that worked. In that case I would say to check out your drivers and make sure your cards are compatible. One more thing, are these hooked up to a router or some network device that may be blocking based on MAC addresses? I'll try to help as much as I can.
Regards,
Steve