Theorotically it shouldnt. But practically, Yes.
Every packet sent or received is accompanied by an acknowledgement by the receiver. Now if you are downloading something, ack have to be sent for these as well. At the same time if you are uploading heavily, its most likely that these "ack"s will be choked at your computers output terminal and there maybe a delay in these being sent. This will inturn affect the download. All this wont happen if there is some sort of priority based transmission. There is a software called "Cfos Speed" (www.cfos.de) which addresses these issues. But no promises from my side as to how well this software works.