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, selecting 'Close'
does not delete a web part.
From the Add or Remove Programs screen find the
Pentalogic - World Time entry and select Remove
If you installed the Web Part onto SharePoint
manually then you will also have to
remove it manually.