lost of some Drive Memory need reply asap.. pls..

i'm wondering what happened to my memory... the first time i checked the
memory this day, i have 39GB used space and the capacity is 81GB. then i left
my laptop turned off when i go to work.. and opened it when i'm back home..
and when i check the Drive C: memory.. i have now 30GB Used space.. i wonder
wer the 9GB go.. and i checked my Files.. and everything is fine... i didnt
do anything else to clean up some memory.. so i'm wondering what
happened..this happened to me twice already.. and the first time..i lost 6GB..

my questions are..
1. Is the Windows Vista Premium has a program that cleans up a disk space
automatically??
2. Does Defragging cleans up a diskspace??
3. is it a cause of a virus?? i already did the virus scan and other stuff
to scan if my Laptop has a virus.. but all are negative... means no virus at
all..
4. What is the most possible thing that causes this??

please.. i wnna know whats happening..

appreciate all the help...;p