What graphics card and drivers did you use previously?  Any new programs installed or uninstalled? Graphics can have an effect if Aero is running, sure; but for the most part a better CPU, RAM and maindrive will affect OS operation more.  For instance, recently I have been deleting whole gigabytes from my drive and I haven't done a defragmentation, afterwards everything except games suddenly sped up.

What video card did you have before?

A graphics card shouldnt have any effect on Windows Performance outside of Aero like Lephron mentioned. There are few possibilities for the increased performance though.
 
1) Simplest explanation would be a buggy and/or corrupt AMD driver that was mucking things up.
2) Some boards are known to have microcode issues with certain models of graphics cards. Maybe yours didnt like the previous card. This one I dont entirely understand but has been documented on the Geforce forums with 400/500 series cards.
3) Windows 8 has some GPU acceleration improvements but I dont see how it would apply to normal operations. I know you can enable gpu acceleration in Chrome and Firefox for loading media faster.

Although it's a good thing that happened, it's rather confusing. But anyway, I just installed a new SSD so now Windows is running even faster than before. So my original question doesn't seem to matter any longer.