How to Reduce primary partition drive

I'm running Windows XP (SP3) with two partitions using Aconis Disk Director 10.0. The primary partition is 45 GB size with 35 GB free. I would like to resize this partition to take @ 20 GB and use for another OS. The software will not allow me to size it down stating there is no free allocated space to use.

How does one go about doing this?

 

Hello shonadle,

Thank you for using Acronis Disk Director 10 Suite

Please look whether this partition has red "C" mark over it in Acronis Disk Director. It means that the partition has errors and the program can't perform resize operations with it. You have to check this partition.
You can invoke system hard disk checking tools directly from Acronis Disk Director Suite by selecting Disk -> Check or clicking Check the selected partition for errors toolbar button.

or you can do it manually:

- Go to the Command Prompt (Start -> Run -> cmd)
- Enter the command: "chkdsk DISK: /r/f"
where DISK is the partition letter you need to check. Please note, that
checking the C: drive may require you to reboot the machine.

Thank you.
Daniel Pustovoy