A+: Windows Minimum Requirements
Any system built in the last few years can easily achieve the hardware requirements needed for installing Windows Vista or 7 and will far surpass the requirements of Windows XP. However, in the real world, digital dinosaurs that might not be fast enough or have enough free disk space to support some versions of Windows still roam the earth (and for those, there's Linux).
Minimum official specs
CPU - RAM - HD Free - Other
7 (64bit) 1GHz - 2GB - 20GB free - DirectX 9 video, WDDM 1 or better
7 (32bit) 1GHz - 1GB - 16GB free - DirectX 9 video, WDDM 1 or better
Vista 800MHz - 512MB - 15GB free (20GB partition) - DVD or CD drive
XP 233MHz - 64MB - 1.5GB (2GB partition) - DVD or CD drive
2000 133MHz - 64MB - 650MB (2GB partition) - CD or floppy drive
Now, those are the 'official' specs. A recommended guideline for Vista is a 1 GHz processor for all versions, and 1 GB of RAM plus a 40 GB HDD for Home Premium/Business/Ultimate.