OptiPlex 780; Very slow internet speeds after installing Windows 7 Ultimate

I just installed Windows 7 on my Optiplex 780, everything went fine, but for some reason my internet speeds slowed down to ~2.5mbps when I was getting ~20 mbps on Vista Business. My upload speeds have increased from ~1.5mbps to ~3.5mbps. I've done countless searches and found no results that help me. I've upgraded every imaginable driver, although I still can't find the PCI Serial Port driver or the PCI Simple Communications Controller driver. Is is possible not having these drivers installed has slowed the internet speeds? If so, where can I find these drivers, or where is it listed in the drivers & downloads page? Or could something else be causing this problem. Besides this one problem, everything is running much better on Windows 7 than Vista ever did. I'm just lost as to what is causing this problem, I've done everything that I know to try and correct this problem but it remains. I even checked with my ISP and they aren't showing any problems on their end. Any help or advice would be greatly appreciated.