Internet speed lags....

I have broadband http://en.wikipedia.org/wiki/Broadband internet (cable) and my question is why is it when I leave my pc on all day or night my internet connection gets slow, but whenever I restart the computer the internet gets noticeably faster... I mean its like a night and day difference. if the pc has been on all day I might download at 100kb/s but if I restart it and download the same file from the same place Ill get like 300kb - 400kb+. And web pages http://en.wikipedia.org/wiki/Web_page load faster too, it may take 10-15sec to load yahoo compared to 2 or 3 sec. Why is this?