vista problem ( hdd size increased)

we are having one user with vista in our company . it is a ibm pc with 150 GB

Hdd . recently we found out that the hdd is showing 130 gb being used and 20

gb remaining . but in fact the actual data is only 20 gb. i used kaspersky ,

windows onecare , adware to check if there is any virus it found some and

deleted it .

i then formatted the harddisk and then it was normal with proper antivirus

updates . again within one week the size of the hdd started increasing . i

used kaspersky online scanner 20 virus found ,so i used the trial version

in the ( kaspersky anti virus )of 30 days to find if there is any virus.

it didnt detect any virus.