Memory problem with vista home edition

Hello,
i've installed Windows vista today. After successfully installation of divers and operating system, i updated my Anti virus (Avast Antivirus 4.8). Then installed Nero 7.0
when i was scanning all the drives, i realized that memory in the system drive(C drive) was continuously dumped down. If i open other drives, memory in C drive continuously lowered. Right my it shows 22GB memory consumption. If i select all the folders, files in the C drive and check the property, it shows only 10 GB memory use. But in my computer, memory used in C drive is 22 GB. I've deleted all the temporary files. Can anyone help me to sort out this problem?Please tell me if i've missed any settings.