If the XP install would not let you make the drive very large, you most likely had it partition it as a FAT partition. I think there is still a limitation for the system drive being on a FAT partition.

Anyway, there are several options available to resize your partition. Partition Magic would be the best, but it is a commercial software package. You could also change the disks to dynamic disks and mount the larger partition onto the C drive. Microsoft also has a utility called diskpart that allows you to resize partitons that I think might be in one of the resource kits. With any of the above, I would convert to NTFS first if it is in fact a FAT partition.