ProcessExplorer takes over when the [ctrl] [alt] [del] task manager leaves off. Process Explorers originally from Sysinternals provides graphical details of what's happening inside your machine. Its free, lightweight and will provide clear insight as to which processes are eating your machine. Each of the graphs provides a rollover tooltip with the process name causing the peek. This graph shows the cpu consumption of a screensaver recently deployed across our organisation. The large drop in cpu usage is when the user logged back in. Having Flash based screen savers such as shown here is a really bad idea; power consumption goes up and all those idle time processes become work time activities.
For C: and other disk drive space; find out what is eating your HDD is made clear by WinDirStat. Originally this drive visulisation tool was created on Linux but this free port delivers a reliable way to find what is eating your hard drive. The interactive bump map shows by size when the large files can be found. Hovering the cursor over a bump will reveal the file path and clicking will show the files place in the file directory hierarchy. Each of the squares of the bump map represents a directory contents.
In this example a data loss prevention utility decided to copy all data sent external hard drive to a location in /WINDOWS/vendor/software name/temp directory location.
Take ownership of your own PC know what happening inside with these useful free system visualisation tools. Other tools available for interior PC examination can be found on this link.