My Thinkpad T40 Laptop - CPU 1.5, 2GB RAM, Windows 7 Build 7048.
The first week of browsing and booting was great. After loading quite a number of applications, the slow-down occurred. Even Firefox ran much slower. A few unnecessary services were stopped, but no real speed improvement.
Then I checked my user profile. It had grown to 1GB!
I immediately moved all my downloaded files from the c:\user\"name"\downloads folder directly to a "new" C:\download folder. I also moved other saved docs and music directly to "new" folders on the root c: Now my profile is just around 100MB. (This is cool if you are the only PC user. But, you could apply security on these folders on a shared PC). Booting and browsing is speedy again!
Hope this helps.