I need some suggestions for my system, about Speeding Up Ubuntu

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http://askubuntu.com – My computer (Sony Vaio VPCF22S1E) has Inter Core i7 (2GHz) processor, 8GB RAM (details) . There are 2 operation systems on my PC . Windows 7 and Ubuntu. My computer's performance was great, a few months ago. But now it's very slow. Windows is faster than Ubuntu, but still Windows is really slow, too. Actually, i'm not using Windows 7. I'm using Windows 2 or 3 hours in a week (I'm studing Computer Engineering and sometimes i need it, but generally i'm working with Ubuntu) . I think there are a few applications which cause slowing down Ubuntu . My question: How can i list all applications wh (HowTos)