How to Format SATA HDD?

Hi,
I have installed in my new computer Windows XP Pro with 50 GB partition. I want to create an extra partition using disk management tools. I cannot complete the formatting for unknown reasons.
After the process, I cannot launch windows anymore. I have ASUS P5Q SE PLUS LGA 775 Intel P45 ATX Intel Motherboard and HDD Seagate Barracuda 7200.10 ST3250310AS 250GB 7200 RPM 8MB Cache SATA 3.0Gb/s Hard Drive – OEM.
I am forgetting to do something or install something?
Any inputs will be highly appreciated.
Thanks.
