I read in the NY Times of a new software product specifically designed to analyze PC slowdowns and detect the causes.
Here is the link:
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If some of you experience slowdowns, this product is supposed to find the causative processes and then the user would have the knowledge to fix it.
At the moment this product is in beta test mode - NOT finalized yet - and only analyzes slow boot ups. Since I don't prefer any beta software, I haven't tried it myself yet.
Here is the link to the NY Times review of the product: