If your wireless isn't protected by WEP or WPS, then it should be. Protecting isn't hard, so if you need help you can just ask. Also, your cpu could be storing to many cookies which can make surfing the web extremely slow ; things such as pictures and videos take up a lot. I mean you can watch a full video on YouTube if it fully loads even if you have no internet connection. Even toolbars slow down your internet.
A free program like CCleaner is great at removing cookies, freeing up RAM, and even cleaning up what uninstalled programs leave behind. Online sites sometimes do what CCleaner does but with less access, since it wouldn't be smart to give a website full access of your computer.
http://www.piriform.com/ccleaner
ADSL uses telephone wires correct? It uses information frequencies not used by a telephone call. Wether your connection uses a DSL filter or is a fiberoptic type of DSL I do not know. But the problem with DSL is when phone lines are being mostly used for calls and not cpu information making hard to reach the normal Internet Protocl Network
Bandwidth is a measurement of data in bites per second. It can also be in Hertz, but that is another type of bandwidth that has to do with frequencies. The peak transfer rate is written on routers and modems as adveristing (peak bit rate). It doesn't always mean it will actaully always work as fast as it says.
Sorry if I confused you or if this doesn't even help at all. And I agree with the guy above me, checking with over computers is a great way to see if it's the connection, computer, or the modem/router that is having the issue. A virus could cause this.