http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx
Look to see what process is consuming your resources. Yes I know it looks intimidating, but it is just displaying a lot of information on a single screen. Just focus on the data that is important to determining why your computer is running slower than it should be and you'll be fine.
CPU usage is usually the most common reason other applications get slow. If it is your Browser that is using a ton of CPU, try disabling plugins.
Memory usage is the second place to look. If you do not have any available physical memory, Windows will have to take application data out of physical memory and write it to the hard drive while it loads the active application's data from the hard drive back into physical memory. There is a special file (page file) that is used for this memory swapping. Your hard drive is way slower than your physical memory, so this swapping will cause a great lag. If you are running out of physical memory, determine which process is consuming your physical memory. Adding more memory is usually a cheap performance boost.
Disk I/O (input/output, or writes and reads) if you have an application hitting your hard drive constantly, everything else will slow down. Your hard drive can only do one read or write at a time. if it is busy doing something else when a process needs to read or write to the drive, that request gets queued and the process waits. Frequent waits can be caused by a misconfiguration in application settings, virus/malware/etc or a hard drive that is badly fragmented. A fragmented drive would be like reading a book where every sentence is located on a different random page and you would have to keep looking at an index to determine which page the next sentence is located. Running a defragmenter will place all the sentences in a contiguous order on the drive, causing fewer index look-ups.
Another good utility to clean out the garbage your computing habits have gathered is CCleaner Free (first 'C' stands for crap).
https://www.piriform.com/ccleaner/download/standard
The same company also makes a hard drive defragmenter:
https://www.piriform.com/defraggler/download/standard
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