All speed "test" sites are rubbish, whatever anyone else on here is about to tell you. The reason is that all they are measuring is the current speed of data between themselves and your machine, not your actual download CAPACITY. Imagine if only one browser was using e.g. speedtest.net compared to, say, a couple of thousand. In addition, there will typically be upwards of a dozen servers in between your machine and the speed "test" site that the IP packets have to hop over. All it takes is for one of them to be experiencing higher than normal stresses and the figures you get will be way off the mark.
The only true way to test your download capacity is to download several largish files at the same time from a server as close to your ISP's network as possible until your bandwidth is completely saturated, and then add their individual speeds together.
Also, make sure that you understand the difference between bits and bytes. Typically, Windows shows download speed in (kilo)bytes, but the broadband industry tends to measure throughput in (kilo)bits or (mega)bits.
Therefore, if you've got a "20 meg" line, that is supposed to give you *up to* (and that's also important!) 20 megabits of download capacity. That equates to 2.5 megabytes of download capcity:
http://www.easycalcul...dwidth-calculator.php