Defragging will increase speed, but theres no need to do it every week, the displacement of the byte types on the HDD is too small to have a recognised effect for at least 3-4 weeks, and thats if you use your computer a LOT.

Slow browsing may be the network, like I say just check your Folder Options and check Automatically search for networks and printers is unchecked. Other than that, it may just be so clogged up with internet junk and proggys that it just needs a good format, I format mine like every few months just to keep it running smoothly .