Extend primary partition - compressed drive?

A friend has an OEM formatted PC with 250gb drive in 2 partitions, 30 gb and
200 gb...

We've unallocated 20gb from the 200gb partition, but "C:\", the primary
partition cannot extend, while the now 180gb "D:\" can indeed extend...

I assume the reason that the primary partition cannot extend is because the
drive is a "compressed drive" with only about 3gb space left (hense, the
reason to extend)...

1. Can this be the reason that the primary drive cannot be extended with the
20gb unallocated space?
2. If I uncompress c:\ drive, how much space might the need (meaning, is
there danger of not having enough space to uncompress?

Thanks