You don't say what speed your cable company says you should get; many offer different speeds at different prices. But whatever speed they can support, what you actually get for a download depends on many factors that are beyond the control of the cable company, as was alluded to in Ibrahim's post. You probably have a car that can easily go 100 mph; but when you are in rush-hour traffic, you may do good to average 25 mph. Is that the car's fault? No.
But first, are you sure the the speed was 100 kb/sec (kilobits per second) and not 100 kB/sec (kilobytes per second? If you got the figure from the Windows download panel, it was the latter -- so the speed was really 800 kb/sec.
But this speed, as Ibrahim indicated, is not a measure of what speed your cable connection can support, but is mainly the speed that the server from which the file was being downloaded can support -- and as this depends in part on how many users are accessing that server at the moment, it can vary with time.
Within the past hour I downloaded a file; it averaged 68 kB/sec, equivalent to 544 kb/sec -- quite slow, as I have about the fastest cable connection available. I immediately ran about a dozen speed checks -- from the east coast, the west cost, the gulf coast, the upper midwest, and even Honolulu. All indicated speeds in excess of 5000 kb/sec, except for servers in Los Angeles and Honolulu, which indicated over 3000 kb/sec. Therefore, the slow download was not the fault of my cable connection, nor of heavy internet traffic, nor of distance. It was due to the server from which the file was being downloaded.
As Ibrahim said, don't judge your provider on the basis of random, uncontrolled servers -- use real speed test servers. He listed several test outfits.
The best, in my opinion, is dslreports.com (now renamed broadbandreports.com) because it gives you access to oveer 300 test servers worldwide -- including my personal favorite, Speakeasy with test servers from coast to coast.
The bottom line is that you probably are getting from your cable company what you are paying for -- it is just that some individual sites can not provide the speed that your cable company can accept.