hey, im having the same problem.
ive got a gigabyte z87x-d3h motherboard. i get similar download speeds to you on my laptop (26 down, 4mb up) but on my desktop i get 4 down, 4 up.
im connected over ethernet. im connected to a powerline adapter. this adapter WORKS FINE. also i used to have a wireless stick in this computer but i moved it to another computer... and guess what. in this computer it was just as slow as powerline. in the other computer its normal speed. so its not my powerline adapter. and on my old computer the powerline adapter worked fine.
im connected with that ethernet cable and on status it says 100mbps. is that ok? im using an ethernet cable that came with an old home hub (bt home hub 3, yellow cable). ive now got a bt hub 5.
im pretty sure the problem is with my computer. ive tried virus scans, installing os again, removing wifi adapter, ethernet (powerline) & wifi. but which component is causing it? i would guess motherboard but i dont want to buy a new motherboard and have the same problem... but i dont understand why this is happening i mean its only download which is affected.
im using win7hpsp1(x64), i7 4770k, palit gtx680(4gb), seagate 1tb, crucial ram 8gb ballistix, coolermaster gx psu. and a coolermaster case. and a samsung/tsstcorp dvd drive. motherboard is a gigabyte z87x-d3h
also the download speed on desktop varies from 0.5-18mb. but usually stays around the same as the upload (4-5). also 4mb download SHOULD be enough for a skype call but it lags like hell.