Extending a volume/partition.

I bought a laptop with Vista(Home edition) pre-installed. The disk volumes initially was C:Vista and Eata. Since Vista would not support drivers for my GeniusHRX scanner so made the laptop dual boot after partitioning E:. But now i do not have enough space left in D:Windows Xp. Using Vista 'Disk Management' i Shrinked E: by 10GB then tried to Extend D: volume but field remains grey. Also used 'Diskpart' at Command Prompt but message is 'Not enough disk space' although it shows 11GB(Unallocated) in total. Need help to extend Volume D: space by 10GB.