A lot depends on the hardware between your router and the isp. The further you are from a node, the slower your connection. Also there are sometimes bad connections on bumps, you'd have to run a ping test to see if your routing is actually good and not suffering lag at other junctions. There's also local traffic that'll change possible bandwidth, a test run when everyone in the neighborhood is online will show slower results than in the dead of night when ppl are asleep.
Windows has had some major upgrades, which have affected audio, graphics, USB and Lan drivers, so if you have older chipset drivers, they could be in conflict, check your motherboard support site for updates.
You could also be suffering interference, RF mainly, from a bad ground in any equipment, including splitters and/or amplifiers on the house. Splitters alone can cause massive drops if there's more than one, and it should be a decent splitter, not the uber cheap variety. The wiring itself might be suspect, even a single kink in the service wire will have bleed to ground, where certain bandwidths are basically nullified.
There's half a hundred different possibilities, sorry, and may not have anything to do with your pc or the router, or they might. Kinda impossible to label just one until you personally can rule them out.