After struggling with my Windows 7 grinding to a halt over the last few months, I finally managed to get it up to speed again. Although some of the suggestions below may seem obvious, I've selected them from a longer list as my recommendations for the four best ways to speed up your Windows 7 installation.
Disable the taskbar "jump lists" to see a small improvement in the responsiveness of the taskbar in general. I never open "recent items" from a jumplist, mainly because I keep my favourite applications (mIRC, Thunderbird, my media player, Notepad++ and Firefox) open at all times anyway.
Disable Explorer's "homegroup" to yield the biggest speed benefits in launching Explorer instances. I don't need to interface with two system services to choose where to save a photo from imgur.com, thank you.
Delete everything in C:\Users*\AppData\Local\Temp that's older than two days and not currently in use. I don't know whether my hard drive is particularly badly fragmented but I noticed an immediate and significant improvement in general responsiveness.
Let me know if any of these help you!