Left 4 Dead requires a little more hardware than HL2..
How "fine" can you run that? High settings? Than you should be fine..
Low settings? Definitely not :/
I think your video card can't handle it.
I can run HL2DM on high settings, the settings even recommend them and it doesn't lag.
QUOTE(Votkrath @ Dec 18 2008, 07:31 PM) Celeron Processor? 768 MB of RAM? An old graphics card from the GeForce 4 series?
Well, basically you can, but not on higher settings. And since you have a 15" monitor you play on a low resolution anyway so I don't know. Try to download some kind of demo or pirate it and see if it works well and if it does go out and buy it. Probably it should automatically set the settings to what fit your computer. If it doesn't work, try to set them even lower if possible and if that doesn't work, no sorry.