I put together a build for only $214 (US) that will run Windows 7 (64-Bit) pretty well. It has plenty of power with 500W, 2.7GHz AMD Sempron processor with a possible second core to unlock, 2GB DDr3 RAM, 320GB 7200RPM hard drive, and a GT 220 512mb video card. This is over the minimum Windows 7 (64-Bit) requirements of 1GHz+ processor, 2GB RAM, and 20GB hard drive space. The BIOSTAR MCP6P3 motherboard also comes with integrated NVIDIA GeForce 6150 Graphics Processing Unit, so you could do without the video-card so long you are not going to play any games or watch any high resolution movies.
Also note that this price could be knocked even lower if you find a slightly cheaper hard drive (with less GB), a slightly lower power supply, if you have an old computer case laying around, or if you choose to just use the motherboard integrated graphic instead of purchasing a video card.
1. Computer Case:
Linkworld 431-08 c8888 Computer Case
2. Power Supply Unit (P.S.U):
DYNAPOWER USA ADW-500 500W Power Supply
3. A Processor:
AMD Sempron 140 2.7GHz Socket AM3 45W Single-Core Processor
4. A MotherBoard:
BIOSTAR MCP6P3 Micro ATX AMD Motherboard
7. DVD – Drive:
LITE-ON Black IDE DVD-ROM Drive Model iHDP118-04
8. Video Graphics Card:
ECS GeForce GT 220 NGT220C-512QZ-F Video Card – Optional (Included makes build = $266)