Firstly delete the web part from any pages you have
added it to.
Note - in order to delete a web part you have to
be editing the Shared view of a page and select
Delete, not Close.
Close
does not delete a web part, it just hides it from view.
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From the Add or Remove Programs screen find the
Pentalogic - PivotPoint Web Part entry and select Remove
If you installed the Web Part onto SharePoint
manually then you will also have to
remove it manually.