The 1.8Gb is your windows swap file to all intents and purposes. You need to click on Tools > Folder Options > View then view hidden files and folders.
Also go into a web page, Tools > Internet Options > Temporary Internet files > Settings , see what it's set to.
It's going to likely be one of the above that's causing it.
Got to a location
http://www.tucows.com and search for something called Treesize. THis runs and tells you what size all your folders are (including hidden folders etc) you'll then be able to see where it's all being eaten up
Yuo can in Explorer got to each folder in turn, right click, properties and it will tell you the folder size. Will take a bit of time, but you'll get tehre by the process of elimination.
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