The theoretical maximum download transfer rate of a modem, in characters per second, is approximately equal to the connection speed of the modem times 0.1125. This means that a 28800 connection should allow transfers of around 3200 characters per second. Otherwise, there may be a bandwidth problem either within the PC or the remote host that is making transfer performance worsen.
Software settings can also definitely be a factor.
Make sure that the connection speed is in fact as fast as you think it is. If you have a 33.6 KB or faster modem, you are most likely not connecting at the theoretical top speed of your modem. It is common to see low bandwidth on downloads retrieving a file from a very busy remote host. If you are trying to download a huge file from a popular site, you are most likely going to see bandwidth reductions caused by network traffic. To eliminate this possibility, try doing a download of a local file from your own account and gauge the download performance that way.