post stands for 'Power On Self Test'. Basically its the computer checking everything is alright and working.
This problem I'm sure is something we've had before where the video drivers have not been installed right, or the res has been set too high for teh montior. After the start up logo, it would then go black...
It will work in Safe mode though. Press f8 at bootup and go to Safemode. it should now boot because it uses default windows drivers for the video.
Now, go into the display properties by right clicking on the desktop and going to Properties, then Settings. Check that the resoultion is for instance at 1024x768 (this is standard) and the colours are set to 16 or 32.
You migth ahve also installed a faulty graphics driver or somerthing weird has happened. To fix this, you'd have to go into the Device Manager:
Find this by going to Start, Control Panel, System, Hardware tab, then click on Device manager.
Then find where it says Display Adapters and check taht there isn't a question mark or an X through the image where your video card is.
If not, still right click on the video card and go to Properties.
Go to the Driver tab and then click on Roll Back Driver if it was working before and isn't now. This will then go back to an earlier version that did work.
If it says there was no previous driver, go to uninstall it, and then go to the manufacturers website of the card and download an updated driver. Then click to install this, and run through the installation process.
Restart, and hopefully everything will be going again