The most common cause reduction of free disk space is the increased number of installed programs. Some applications can take a very significant amount of disk space. For example, dictionaries, databases, image editors, games. With a stable Internet connection all antivirus programs automatically update their virus database, so the free space fills up quickly.
If the user is interested in collecting pictures in high resolution, movies, music, it is likely they are just copied on the system drive. First, if these data are downloaded from the Internet (websites, file sharing, ftp servers), the default is all browsers save information in the Downloads folder, which is located in the My Documents folder, and then, with each new file the disk space is reduced. Secondly, all downloaded from torrents also immediately gets on the C drive.
Temporary files are accumulated in the process of operation of OS and gradually require increasing space. For example, the Temp folder is able to "swell" up to a decent size and contain, in fact, unnecessary or irrelevant information. It often get the installation files for MS Word, the distributions of the previously installed programs etc. Internet Explorer is also automatically stores web data, which are contained in the folder Temporary Internet Files.
System files support the performance of the operating system and help to restore its functionality in critical situations. For example, if the driver of any device is updated unsuccessfully, then you can "roll back" the OS to the last good checkpoint. Naturally, these restore points are stored in files and occupy a certain place, which over time only increases. Another disk holds the paging file pagefile.sys. With the help of the moments of the limit load, the computer will run faster, but the number of free gigabytes on your system drive will be reduced.
The most dangerous cause reduction of free disk space With are viruses. Some of these programs are able to place their files in the system directories and imperceptibly for the user to consume all available system resources. Viruses can create your own folders in which there is an accumulation of various records, punctures and other files malware. Thus disk space will inevitably be spent.