Windows 7: HDD Problem [Size Up and Down]

i don't know how it happen. i just start my computer normally.
and my hardisk size is normal, then a few minutes later i lost all the free space.
and this problem only at my C, system is in C. after restart, there still no change.
then suddenly the free space increasing, and then decreasing again (free space size is decreasing every minute until the disk C is full).

is this hardware problem??? or maybe a virus, malware, or even spyware???
btw, i'm using 7 Professional and for antivirus i'm use Microosoft Security Essentials