The adslguide speedtest that Freak is referring to is here
It's uses a java applet so you may need to update that to run the test. It's a reasonable guide for your speed but it might not be quite what you expect as there are overheads that eat bandwidth.
For example, my 1MB connection should in theory be 1024 kilobits/second. When in reality a good result is more like 960 kilobits/second.
For accurate results I would stop all your browser windows, except the test window obviously, and all other apps from running. Especially p2p software packages like Kazaa etc! Oh and your firewall will slow things down, especially if it supports SPI.
Also, if your results are poor - run your anti-Spyware software before you ring your ISP. These things eat your bandwidth. If you don't have any try LavaSoft Ad-Aware and Spybot Search and Destroy. I use both and update them regularly.
Hope this helps ...