I imagine that anything (games, programs, apps....) installed with .dll, .ini, drivers, etc...files will put put one or a copy in the windows directory. I can see it growing pretty fast.
I always used Partition Magic in the past, but I don't think we need to in V64. If you go to Computer Management/Disk Management, it now offers Shrink or Extend volumes. I haven't used this yet, but I assume if you shrink D for example, you can expand C. But as I say, I haven't tried this yet. So backup first.
While you are waiting for someone else to post their experiences with this, you might like to read this tutorial on using the shrink/expand volume option in Vista: