Use wget - free little program download is on website link to the left

Here's what you can do: 1. copy the 'filename.blah.part' to the wget directory. This is needed because Firefox keeps the file open, keeping wget from writing to the original. 2. rename it to 'filename.blah', removing the .part extension 3. run 'wget -c http://site/file.blah' 4. Wait. If the download stops again, run the 'wget -c http://site/file.blah' command again.
You can get wget for Windows here. http://users.ugent.be/~bpuype/wget/