Hi Sue,
What model Mac do you have, how much memory and how much drive space do you have? Command click on your harddrive and click get info it will show the drive size and avaliable space. If it is less then 25% of the toal size you will experience slow downs, as stated in the above post.
There could be other things at play, but this is the starting point. BTW, do you leave your computer on all the time or do you shut down regularly?
Twist
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Everything will be OK in the end.
If it's not OK, then it's not the end