I guess my first question would be - is
every phone that is connected to that phone line going through a DSL filter, even wall-mounted phones. Each phone must have a filter and then your computer should actually have a dual filter and your computer must be attached to the side labeled for computer (a phone can be plugged into the other side). Without these filters you connection will cause data loss and even loose connection completely. Most providers give you 4 or 5 in the installation kit but if your house has more than that then you can buy them from your ISP for about $15 each or find them at a computer store or online to save a few bucks. Without a clean line data often must be retransmitted multiple times just to get one clean portion of a file (called a packet).
Second, what level of DSL service have you contracted with your ISP Most ISPs offer different tiers, the slowest probably being 768kbps. Consider upgrading to a more expensive plan.
You can test your speed
HERE