Download: TreeSize Free Portable and run.
Look under C:\ProgramData\Microsoft\Search\Data\Applications\Windows, there is a file Windows.edb. Since you've just upgrade to 8 from 7, Windows might be busy rebuilding the search index. Check the filesize, if it is large then just disable the Windows Search service and delete it, let Windows rebuild the index.

Also, look under a hidden folder under C:\System Volume Information to see how big the space it's taken, you might have some old system restore points carried over from Windows 7 and can cause corruption.