My video cards supports two monitor, I connected the second monitor to it. It shows the green light but it says no signal. When I started the computer up both monitors where showing the windows logo and started XP. Then it went blank but the green light is on. Please help! Thank you.

Did you make sure your video card is configured for dual monitor and also if it is extended desktop or copy desktop.

Check these settings out on your one that does show

1.Right-click the desktop, and then click Properties.

2.In the DisplayProperties dialog box, click the Settings tab.

3.Click the Display list and select your external monitor. If you do not see multiple monitors listed, your computer hardware may not support Dualview. You can still use MaxiVista to configure a second computer as an additional display, however.

4.Select the Extend my Windows desktop onto this monitor option. Click Apply or OK.

Also
http://support.microsoft.com/kb/307873