Windows Server 2003

Web Edition
Supports two processors and 2GB of RAM.
133-MHz processor (550 MHz recommended)
128 MB of RAM (256 MB recommended; 2 GB maximum)
1.5 GB hard disk space

Standard Edition
Supports four processors and 4GB of RAM.
133-MHz processor required; 550-MHz or faster processor
128 MB of RAM required; 256 MB or more recommended
1.5 GB hard disk space

Enterprise Edition
Supports eight processors and 32GB of RAM.
133-MHz or faster processor for x86-based PCs; 733-MHz for Itanium-based PCs
128 MB of RAM required; 256 MB or more recommended
1.5 GB hard disk space

Datacenter 32-Bit Edition
Supports 32 processors and 64GB of RAM.
400 MHz processor required; 733 MHz processor recommended
Minimum: 512 MB of RAM; recommended: 1 GB of RAM
1.5 GB hard disk space

Datacenter 64-Bit edition
Supports 64 processors and 512GB of RAM.
Minimum: 400 MHz processor for x86-based computers or 733 MHz for Itanium-based computers; recommended: 733 MHz processor
Minimum: 512 MB of RAM; recommended: 1 GB of RAM
1.5 GB hard disk space

Windows2000

Windows 2000 Professional
Supports up to two processors and up to 4GB of RAM.
Pentium 133 MHz, 65MB of RAM, 650 MB free hard disk space (2 G recommended)

Windows 2000 Server
Supports up to four processors and up to 4GB of RAM.
Pentium 133 MHz, 128MB of RAM, 1 GB free hard disk space (2 G recommended)

Windows 2000 Advanced Server
Supports up to eight processors and up to 8GB of RAM
Pentium 133 MHz, 256MB of RAM, 1 GB free hard disk space (2 G recommended)

Windows 2000 Datacenter Server
Supports up to 32 processors and up to 64GB of RAM.
Pentium 133 MHz, 256MB of RAM, 1 GB free hard disk space (2 G recommended)

Windows XP

Home
Supports 1 processor and up to 4GB of RAM
233 MHz CPU (300 MHz recommended)
64 MB of RAM (128 recommended)
1.5 GB of available hard disk space

Professional
Supports up to 2 processors and up to 4GB of RAM
233 MHz CPU (300 MHz recommended)
64 MB of RAM (128 recommended)
1.5 GB of available hard disk space

64 Bit Edition
Supports up to 2 processors and up to 64GB of RAM
AMD Athlon 64, AMD Opteron, Intel Xeon with Intel EM64T support, Intel Pentium 4 with Intel EM64T support CPUs
256 MB of RAM
1.5 GB of available hard disk space

Windows 95/98/ME

Windows 95
Supports 1 processor and up to 2GB of RAM
x386 CPU
4 MB of RAM (8 recommended)
55 MB of available hard disk space

Windows 98
Supports 1 processor and up to 2GB of RAM
66 MHz CPU (300 MHz recommended)
16 MB of RAM (24 recommended)
225 MB of available hard disk space

Windows ME
Supports 1 processor and up to 2GB of RAM
150 MHz CPU (300 MHz recommended)
32 MB of RAM (64 recommended)
320 MB of available hard disk space

Yeah, thanks for the list. It's interesting to see that windows can run on pretty much any computer made in the last eight years (except for Vista)