c: drive automatically full

i have 30 gb c: drive partition, and using windows 7 alongwith other softwares....

from 2 -3 days it become automatically full, but when i see the properties of all folders in it and calculate it, shows only 18 gb, while my partition remains only 54 mb.

i have also used treesizefree software and it shows 18 gb data and 54 mb free.
one interesting thing is that free size is not remains always same, sometimes is 1 gb free, sometime 633 mb and some times 54 mb.....


pls help.... so that i don't have to format that partition.............

thx a lot sir,

i applied the same setting, and it works and problem solved

thanks again.....................
ranjit