Partitioning HDD in vista

Hi guys
I bought a dell laptop with 320 GB HDD and the OS is Vista Home Premium.I
have tried to make c drive smaller and make another drive, i have used
partion manager, partition commander and the shrinking feature of vista
itself but none of them wokrd, partition magic and partition manager resize
my c drive but when i want to create a partition they tell me that there is
an error and i should use a 'Repair Tool' to fix the problem. Shrinking
feature of vista says that "The attemped operation is invalid.Either the
parameters specified are invalid or the operation cannot be completed on the
selected object. Refer to Disk Management help for assistance on the correct
use of the attemted operation". I would apperitiate if someone could help me.

Cheers