Why does my external monitor do this?

Out of boredom, I’ve plugged in the old monitor from my desktop into my laptop to use as a second monitor. However, it only displays picture for a second or two before going blank again. The power indicator on the monitor says that it thinks it’s connected to a powered-up computer. My laptop’s monitor is working fine, otherwise I wouldn’t be typing this from it. What can I try to fix it?

Specs, anyone?
Operating System: MS Windows Vista Home Premium 32-bit SP2
Processor: Intel Mobile Core 2 Duo T5850 @ 2.16GHz Merom 65nm Technology
RAM: 3.0GB Dual-Channel DDR2 @ 332MHz 5–5-5–15
Motherboard: Dell Inc. 0H282K (Microprocessor)
Monitors: Generic PnP Monitor @ 1280×800 [my laptop’s screen]
DELL E153FP @ 1024×768[the old screen]
Graphics: 256MB ATI Mobility Radeon HD 3400 Series (Dell)
I’ve got more if you need ‘em.