Dell 1545
Hard disk

I have bought dell inspiron 1545 laptop a year ago.all the parts r performing quite normally, but if I install any software of about 10 MB the disk space reduces about 1GB.then it never restored even restarted. I have presently 112 GB of disk space, but if I add all the size of the programs in my local drive (C) I get about 152 GB free space but it shows about 112 GB.please tell me if there is a solution to fox this problem.

note: my total disk space is 250 GB and that of local drive (C) is 218 GB.