that the minimum hardware specification for Windows 7 to run is as follows:-
•1GB of main memory
•16GB of available disk space
•Support for DX9 graphics with 128MB of memory (for the Aero interface)
•A DVD-R/W drive
My own view is that this should be regarded as an absolute minimum, and if you want the operating system to run at all well you should have a faster processor and at least 2GB of RAM onboard.