My PC once slowed down to a crawl. The culprit was finally found to be the browser download history file. It seems the browser remembers every file you ever downloaded, unless you clear the list. This download history file loads when the browser starts and went it gets to be large it takes up tons of memory. Once I cleared that up, everything was back to normal. Now I always clear the download history file after I download.

If you delete browser cache, cookies and browser history, it will not delete your stored passwords.

Before I take any drastic steps, I always backup first.

Also, go into Task Manager by pressing ALT-CTRL-DELETE. Go to the 2nd tab, which is PROCESSES. Sort on CPU by clicking it twice so that the jobs that are using the most CPU are on top. Sometimes you can get an out of control job that is eating up all your CPU and slows everything else down. If you recognize the task and know that it shouldn't be doing what it is doing, end the task and see it that helps. Windows some times does strange stuff.