I have a 180Gb disk on a server, which can be seen from Disk Manager
within Windows 2000 - it recognises it as one single partition, C
drive, of 180 Gb.

In Windows Explorer, the drive is recognised as 4Gb only, and the
operating system is telling me the drive is full.

The drive is NTFS

How do I make the Operating System (Windows Explorer) see the full 180 Gb??

The reason is Windows 2000 can only recognise upto 137GB Capacity
Barrier is also known as 48-bit Logical Block Addressing Support for
ATA (IDE), Serial ATA or ATAPI disc drives. The 137GB limitation does
not affect SCSI interface disc drives.
Octek has updated all BIOSes to support 48-bit LBA since 2002 (Code
2K300 or later); however, in order to properly access the full
capacity of an ATA interface hard drive larger than 137GB, your system
must meet the system requirements described below. There are three
general methods of support:

As such you are required to partition your HDD into more manageable
portions, alternatively you can upgrade to WIndows 2003

48-bit Logical Block Addressing(LBA) is supported in Windows 2003,
Windows XP SP1 and Windows 2000 SP4. However, sometimes it is not
enabled by default.
See
http://support.microsoft.com/kb/305098
http://support.microsoft.com/kb/303013