I'm not technically minded so I may have misunderstood the setup here.
You said this computer is hooked up to the internet through a hard wired network, ethernet cable from the router through a Netgear 'wifi' extender. That Netgear wifi extender looks like wireless to me.
Assuming my poor technical knowledge has not let me down, what happens if you remove the wireless connection and connect directly via ethernet cable to the router? It might show immediately whether the wifi is a problem.
If so, then at least you have a start point for diagnoses.
If not, what about anti-virus and anti-malware scans? Since the internet worked well for the first two days or so it doesn't look like a hardware or driver problem.
So I recently built my first desktop. Everything was going fine until my internet started acting up. After a day or two it became unbearably slow. I attempted to download Itunes and the download speed got down to 85 bytes/second. Not megabytes, not kilobytes but individual bytes. Here's the specs of the computer:
Intel Core i7-950 3.06GHz Processor
Western Digital 500GB 32MB cache 3.5" Hard Drive
ASUS Sabertooth X58 Motherboard
3xG.Skill Ripjaws Series DDR3 RAM (12GB of RAM total)
MSI N460GTX CYCLONE GeForce GTX 460 Graphics Card
Windows 7 64bit
I also have the computer hooked up to the internet through a hard wired network (Ethernet cable from the router through a NetGear wifi extender into the computer).
I don't think it's the internet itself as I have taken other computers (two laptops) and disabled their wireless capabilities then hooked them up through the exact same setup as my desktop and the internet was running great on both of them. I also heard that this could be a problem with viruses and spyware. The only sites I visited (Google Chrome download page, Minecraft.net and Ign.com) wouldn't have given me any malware. The programs and games I have on the desktop I downloaded on the laptops and transferred via USB stick. I also trust all the programs (Minecraft, Battle for Wesnoth, winrar and Itunes). Any and all help with this problem is appreciated.