i would like to contradict - less than 2GHZ, less than 1.5GB of RAM, less than a dualcore is a practical no go for Win7.
win7 without ripping your hair out is a dualcore at 2Ghz, 2Gigs of DDR2 RAM and 25 GB for the OS only. Then the programs, then the Data, then the Swap. make it an 80GB HDD
...and removing all the extras that eat up Memory.
Proper Win7 starts above that, in computers with dual 3Ghz, 2.5 or3 GB of 667Mhz Ram and a 120+ HDD... the earliest hardware i would recommend to run 7 would be from around 2007.
The official Numbers are these:
32-bit 1 GHz
IA-32 processor
Memory (RAM) 1 GB
DirectX 9 graphics processor with
WDDM driver model 1.0
(Not absolutely necessary; only required for
Aero)
HDD free space 16 GB
64-bit 1 GHz
x86-64 processor 2 GB
Graphics card DirectX 9 graphics processor with
WDDM driver model 1.0
(Not absolutely necessary; only required for
Aero)20 GB of free disk space
pulled from Wiki.
But they represent the bare iminmum for the install for a fresh Win7. They do not encompass the software that is the true goal to run a Windows, the upgrades, the additional features that are running BESIDES the Win7, such as your antivirus program and stuff.
Do not thrust these figures, but double them, just as much as you double the Hardware requirements for running a game at useful settings.