IE6 scroll bar anomally

This is weird and difficult to describe without demonstrating, but I'll try.

I notice on some long web pages when I single click in the vertical scroll bar area, that IE will advanced (that is, page down) as if I had done two single clicks.  

For example, I can get to the bottom of http://news.google.com with 6 page-down keystrokes.  That makes it 7 sections: the top section plus 6 page-downs.  But when I click in the vertical scroll bar area, it only takes 3 clicks to get to the bottom.  In those clicks, I see sections 1, 3, 5, and 7, and I don't get to see sections 2, 4, and 6.

I do not that this problem with Word, Excel, AdobeReader, or even DOS Edit.  And I don't have this problem with all web pages.  For example, these EE pages scroll properly.  But many web pages do not.

I have had this machine for 2 years with WinXP Pro and IE 6 with all of the latest service packs and updates, and I have only started having this problem this week.  I did have my computer worked on this week by someone else, and through an unfortunate set of circumstances, the machine has been prone to blue screens since then.  However, I still would not expect this scroll bar weirdness.

How can this problem by resolved?