You could try the free Paragon Partition Manager 2005 click here. I've tried it with WinXP & it works, but the partitions I merged didn't have the OS installed on them. The option to merge is under Wizards > Merge partitions.
Should that fail, there's another free tool called GPartEd LiveCD click here, which I've also successfully used to merge Linux partitions, one of which contained the OS. It can be used to merge Windows/NTFS partitions too. G