Here’s a list of how much RAM the various Windows versions and editions support:

Windows NT 4.0: 4 GB
Windows 2000 Professional: 4 GB
Windows 2000 Standard Server: 4 GB
Windows 2000 Advanced Server: 8 GB
Windows 2000 Datacenter Server: 32 GB
Windows XP Professional: 4 GB
Windows Server 2003 Web Edition: 2 GB
Windows Server 2003 Standard Edition: 4 GB
Windows Server 2003 Enterprise Edition: 32 GB
Windows Server 2003 Datacenter Edition: 128 GB
Windows Vista: 4 GB
Windows Server 2008 Standard: 4 GB
Windows Server 2008 Enterprise: 64 GB
Windows Server 2008 Datacenter: 64 GB
Windows 7: 4 GB