hard disk problems

hello
my hard disk space is 40GB, i have two partitions on it, one for linux (25GB) and the remaining for windows (15GB). The problem is that the windows partition space is continously decreasing, without me installing or adding anything that takes space !
for example, i check the free space on the windows partition, and it’s 1GB let’s say. Then the same day at night, i check it again, and it’s 900MB !
could anyone please tell me what’s going on?
thank u in advance