Dark image while gaming

Hi, am having real trouble with graphics brightness whilst playing certain games ie Call Of Duty, during the training when asked to proceed to the firing range through a door i cannot see the door without the brightness settings in the in game video settings being turned on full, the monitor brightness settings are already at max, i have tried to adjust the gamma etc settings but all my efforts just make it worse so i restore the default settings.

System Info:
O/S: Windows xp home (build 2600) with SP1
DirectX 9
Latest Nvidia Driver and all critcal and driver updates from windows update have been installed.
CPU: Athlon XP 2600
Ram: 512 pc2100
G/Card: NVIDIA GeForce4 Ti 4200 (Sony)
Monitor: Proview 17"
Board: ASUSTeK Computer INC. A7N266VX REV 1.xx
Bus Clock: 133 megahertz
BIOS: Award Software, Inc. ACPI BIOS Revision 1002 12/13/2002
Standard floppy disk controller
Primary IDE Channel [Controller]
Secondary IDE Channel [Controller]
Standard Dual Channel PCI IDE Controller
NVIDIA(R) nForce(TM) Audio
NVIDIA(R) nForce(TM) Audio Codec Interface
ST3TIGER SCSI Controller
VOB InstantDrive Controller
NEC PCI to USB Enhanced Host Controller (B1)
NEC PCI to USB Open Host Controller
NEC PCI to USB Open Host Controller
NEC OHCI Compliant IEEE 1394 Host Controller
Standard 101/102-Key or Microsoft Natural PS/2 Keyboard
Sony Memory Stick controller(WB)
PS/2 Compatible Mouse
USB Root Hub
USB Root Hub
USB Root Hub
c: (on drive 0)20.00 GB2.02 GB freed: (on drive 1)122.91 GB51.88 GB freee: (on drive 0)19.99 GB16.28 GB free

Orignal System was a Sony Vaio slightly altered but only with an extra h/d and extra256 of ram i have 1 gig of pc2700 to fit but its not matched to the motherboard but it is Crucial.

Hopefully from my system info you might be able to offer some solutions before i go and buy a new g/card or monitor.

Thanks in advance.Katbloke