[sameer752003]

My NTFS partition on C drive where I have installed OS Windows Server 2008 shows low disk space even when I have not installed much programs. When I checked the properties of individual folders they add up to mere 11 GB. My drive is partitioned at 45 GB. So why this error. I have used Disk clean up utility built in to windows. It could free merely 2.5 GB.Now I am unable to install any programs.

[allen2]

It could be shadows copies, pagefile, hibernate file.....How many free space is shown ?
45GB isn't much for a windows 2008 install if you installed many applications on C:.

[sameer752003]

I checked the file sizes of individual folders on C drive. It adds to 11GB approx. Also I have no huge softwares installed.

[allen2]

So you didn't answer the only question and don't tell if you check what i said so why did you post the same thing again?

[adamt]

There are many reasons why this might be. Firstly, as allen2 has told you, the space may be used up by pagefiles, shadow copies, hiberfil, etc - files which are usually hidden from the shell unless you configure it otherwise.

Then, bear in mind you there's things like reparse points (junctions), compressed files, etc - see: Computing the size of a directory is more than just adding file sizes on Raymond Chen's blog, and the similar Windows Confidential article he wrote for an overview.

[Kelsenellenelvian]

How much ram do you have?

The amount of space you are missing IS most likely the pagefile and hiberfile as others have said.