If none of this works
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Try windows Disk manager. (USE AT OWN RISK). Disk manager doesn't incorporate all the 'saftey features' partition magic has. If you mess around too much you may loose your data and render your PC unbootable, meaning you will have to format the entire drive and reinstall windows. so BE CAREFULL.
To get into disk management
Go Start>control panel>Admin Tools>Computer Management. Select storage in the left menu, then Disk Management. Right click the unallocated space and create a partition.
Also i'd like to know how this problem occured? How did the unallocated 6GB get there in the first place?
Hope this helps.