Wait, so is the system not booting up period or is it not going into Windows? I don't quite understand what DOES happen during boot-up.
First off, check that the 24-pin motherboard power and the 4-pin CPU power connectors are in place.
Then check that everything is plugged in properly (SATA drives connected to mobo, drives have power cables plugged in).
Check that the monitor is connected to the VGA port and that the monitor has power.
If all these are done right, the system should boot up at least to a BIOS screen. From there, you need to install an operating system onto the hard drive and you're good to go.