Your modem is capable of speeds of 50kbps? Are you talking about a cable modem/adsl modem?
You might want to check and reinstall your ethernet card drivers, because that could be the root of your problems. Also, make sure that you don't have an IRQ conflict between your ethernet card and sound card (or any other PCI devices). Just go to Start->Control Panel->System, and look at the properties for your NIC.
Limiting your upload speed doesn't fix the problem that you have, which is what you should be trying to do.