Can not resize my system-primary partition

Hi there,

 

I am using PM8.05

 

My partitions :

 

C - NTFS - Primary - 20gb fullsize - 5.8gb free (SYSTEM partition,winxp )

D -  NTFS - Logical - 690gb fullsize - 100gb free

 

I want to move 30gb from D to C.

 

 What I did is :

 

Resized D to 665gb ,70gb FREE

Resized EXTENDED (primary) to  same as D , 665 fullsize , Used 665

Now there is UNALLOCATED 30gb

 

I can resize D again to its full size , 690 by using unallocated 30gb

 

but

 

I can not add that 30gb to my C partition , Its max is 20gb, I can decrease it to 15gb but can add what I want, 30gb from unallocated.

 

What is the problem ? Thanks a lot