Windows 7: Fluctuating free space on system drive?

Drive space keeps disappearing from my c drive for no reason. I have windows installed on an 11GB partition. In the last week, free space has dropped from around 1GB to nothing, then back to around 300MB. I can't understand why this is happening, I've installed nothing, disabled system restore and hibernation, and have a fixed pagefile of 250MB (all I need). I use CCleaner but none of the missing space is recovered, windows has eaten over 700MB and I see no reason for this, can anyone explain this peculiarity?

maybe temporary files? try doing a disk cleanup.

Do you have a antivirus seems to me it must be some type of virus trojan, malware etc that must be eating up your free space.

P.S maybe try defraging your disk might help

Why would you want to install your OS on such a small partition? How big is your hard drive? Personally I see no need to partition a drive unless you are going to dual boot.

Its not a virus, i clean temp often, browser caches all cleared by CCleaner, and yes this is a dual boot, can anyone think of a logical answer as to why windows would eat disk space for no obvious reason?

The swap file is another typical cause to make freespace vary, according on what you're doing. By default Windows places it on the system drive and allows to grow and shrink at will, making such variation possible. If this is the case, you can set a fixed size swap file to avoid any further problems.
Another cause may be temp files, some programs logs may be writing things too, or simply any program saving data and its configuration. Check if in the appdata folder there is a significant amount of space used.