I may be wrong, but I get the feeling you are confusing some terms when you ask about resume from disk. First let's tackle that just in case.
1. Installing: If you are installing, you are not downloading, it's all "local". You could unhook from the internet and still install from disk and has nothing to do with internet.
2. Downloading: This is you getting information from a server, the server is "uploading" sending you the information. Especially on 56k, it's a VERY slow process. It's like this, if Microsoft has a bag of sand, they are sending you the grains until you have a bag of sand. Hence uploading\downloading.
3. Let's compare DSL or Cable with 56k: DSL\Cable is like sucking up a bag of sand with a shopvac, while 56k is like sucking up sand through a drinking straw.. which one is going to be done much faster? Right.
4. Local: If you have a disk, you put that in your machine, it's local. You are not uploading or downloading anything, no sand. The info goes directly from your CD\DVD drive to your hard drive, no internet. The info is on the disk and simply, when read, expands, copies, installs to your hard drive on your computer.
Now, there is no resume feature with SP3, it installs and is done, but it only takes about 15-20 min, maybe longer depending on how fast your machine is,to install and is quite a large install.
You could stop it in the middle of an install, then wait for it to uninstall, then have to reinstall again, NOT RECOMMENDED.
Hopefully this gives you a better understanding.