Can't update because modem drops out

Using a dial-up connection on XP machines through a SMC router. When I have to download large files, generally those that take over an hour, the connection drops and I have to start all over. I was able to solve most of the problems with a download manager that will pickup where it left off when I re-connect. However, I have not been able to find anything that will work with upgrades. The manager works fine when downloading files but it will not work on things like Microsoft Windows upgrades or driver upgrades. How can I get these critical upgrades? Is there a program like the download manager that will work to restart interrupted upgrades or is there some setting I can change to stop the modem drop outs? I have tried several different modems from US Robotics on down and all did the same thing.