You'll have to give us more details than that if we are to help. Do you have 2 physical drives or just 2 partitions on the same drive? What version of windows? How large are the drives/partitions? I presume the one with windows on is virtually full up.
If 2 partitions, you could use something like partition magic to resize. If 2 drives, and second is bigger, you could put operating system there. Have you 'cleaned up' drive? You could put swap/pagefile on second.
But more details please.