My computer is getting problems and slower? How to fix or Should I get new computer?

Lately like over the last week I feel my computer is getting very slow has several prolems.
This is a list of some problems
1. Unable to turn off computer(fixed)
2. My CPU constantly warming up.
3. Either unable to open email or very slow (my family usee aol and i use yahoo both has this problem on this same computer we share).
4. Similar sites all of a sudden becomes very slow and some are fast.
such as yahoo.com takes a sec.mail.yahoo.com takes forever.

Is there a way to get it to move faster or is it time for me to get a new computer?

Also I have a dell desktop and has had it for 1 and a half year.
Another problem is I am trying to download a aprox 4 mb document where it is taking over 15 minutes and at most it supposively should be taking less than 3 mins normally. The internet speed i have is verizon high speed.

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