Here's something that I just discovered as I'm preparing to wipe my disk & make a clean start.

I was about to backup my Outlook .pst data file (in Documents and Settings\...\Local Settings\Application Data\Microsoft\Outlook) when I was shocked to discover how large it was, despite the fact that I'd cleaned out my email, deleted a whole load of stuff, and emptied the deleted folder.

Then I had a look in the settings & saw the "Compact Data File" option.  It took a while, but reduced the file to 1/8th of it's orginal size. 

To compact your data file, in Microsoft Outlook, go to Tools, Account Settings, Data Files, Settings, Compact Now.