COD4 already has a health bar built into the game. All you do is show it in hud.menu:

Code:
		// HEALTH BAR
		itemDef
		{
			name "healthbarback"
			rect 6 471 124 6
			forecolor 0.7 0.7 0.7 0.3
			background "white"
			ownerdraw CG_PLAYER_BAR_HEALTH_BACK
			visible when( !dvarInt( "wideScreen" ) && !flashbanged() && !ui_active() && NOT_SPECTATING && team( name ) != "TEAM_SPECTATOR" );
			decoration
		}
		itemDef
		{
			name "healthbar"
			rect 7 472 122 4
			forecolor 0.7 0.4 0.0 0.3
			background "white"
			ownerdraw CG_PLAYER_BAR_HEALTH
			visible when( !dvarInt( "wideScreen" ) && !flashbanged() && !ui_active() && NOT_SPECTATING && team( name ) != "TEAM_SPECTATOR" );
			decoration
		}
		itemDef
		{
			name "healthbarcross"
			rect 132 467 12 12
			forecolor 1.0 1.0 1.0 0.3
			background "hud_healthbar_cross"
			ownerdraw CG_PLAYER_BAR_HEALTH_BACK
			visible when( !dvarInt( "wideScreen" ) && !flashbanged() && !ui_active() && NOT_SPECTATING && team( name ) != "TEAM_SPECTATOR" );
			decoration
		}

		// HEALTH BAR WIDESCREEN
		itemDef
		{
			name "healthbarback"
			rect 5 471 93 6
			forecolor 0.7 0.7 0.7 0.3
			background "white"
			ownerdraw CG_PLAYER_BAR_HEALTH_BACK
			visible when( dvarInt( "wideScreen" ) && !flashbanged() && !ui_active() && NOT_SPECTATING && team( name ) != "TEAM_SPECTATOR" );
			decoration
		}
		itemDef
		{
			name "healthbar"
			rect 6 472 91 4
			forecolor 0.7 0.4 0.0 0.3
			background "white"
			ownerdraw CG_PLAYER_BAR_HEALTH
			visible when( dvarInt( "wideScreen" ) && !flashbanged() && !ui_active() && NOT_SPECTATING && team( name ) != "TEAM_SPECTATOR" );
			decoration
		}
		itemDef
		{
			name "healthbarcross"
			rect 99 467 9 12
			forecolor 1.0 1.0 1.0 0.3
			background "hud_healthbar_cross"
			ownerdraw CG_PLAYER_BAR_HEALTH_BACK
			visible when( dvarInt( "wideScreen" ) && !flashbanged() && !ui_active() && NOT_SPECTATING && team( name ) != "TEAM_SPECTATOR" );
			decoration
		}
The built-in health bar is superior to any hud element because when your health is low, the bar pulses and changes color to show low health.