Black screen when i play Half-Life 2

I have a Intel Pentium 4 CPU 2.80GHz, 1024 RAM
running on XP, DirectX10 as my system.
ATI Radeon HD2400pro 512 AGP.
When i had onboard graphics i could play all my Steam games half life, opposing force, Half-Life Lost Coast, Half life 2,all on single player no problems apartr from litle lag. As soon as i tryed to multiplay i got Blue screen. That is What prompted me to get the above graphics card.
I instaled the card no probs and it rips on Opposing Force, Half-Life on multiplayer. but as soon as i tryed to play Half-Life 2 Looked like it was working then at the last moment blackscreen with game sound. Same for Lost coat and few other newer games. At this stage of the fight i was running on DirectX9c and thought that Half-Life 2 and such where DirectX10 games. So managed to find a DirectX10 that works on XP thinking this would solve all my problems. Then realized they are directX9. It is doing my head in please HELP