OS

my C: drive holds the capacity of 48 GB... Used space in increasing by some few mb's then and there...I m not installing any application programs... for ex,if it has 28 gb free today,after some 2 or 4 days, it will show that 27.8 or 27.7 GB free......How this possible..? Even after deleting temp files and prefetch files,it remains same...

Hibernate or pagefile?
I would not consider that difference significant

Restore points use several Gb's. You can clean older ones out if you want to.