My c drive space is decreasing automatically

[moserwa]

Hello,

 

[ksiemb]

Hello,

1. - Run Windirstat to see which folder is increasing in size.
http://windirstat.info/

2. - Might be an infection - Run your AV scan and Malware scans.

 

[wrathofdragon]

If your computer is mostly OEM, than you have one Hard Disk, with a reserved partition for the System Files (if you open Computer, its "local disk D" or something...)
it could be that you have too many system restore points, or you messed with your system and started putting crap on your Local Disk D, or you have a virus (like @ksiemb said) that is changing the directory size of D.