|your computer has no effect on internet speed. The only thing that would effect it would be the NIC and I promise you that your NIC is faster then your internet.|
Mostly likely what is happening is that your computer has so many stuff running, it is slowing the system down. Having a virus can also cause this as well.
I've had major problems with Vista being unstable personally. I would run a virus scan and run an msconfig (Start, Run, Type in "msconfig") to free the system up from having to load a lot of applications at once.
What Browser are you using? You say rolling back/updated drivers and it increased your internet speed, but how are you determining this?
There are a lot of variables that could be causing this, but I promise you windows 7 is not changing your internet speed, the system may just be overloaded from a bunch of applications.