Your video controller is integrated and shares the memory of your system.
The more programs you have running in the background, the less memory is available for the video/graphics controller.
How much RAM do you have? The 2400 is limited as to how much it can have. It has only two RAM slots.
Here is a diagram of the motherboard in the Dell 2400:http://support.dell.com/support/edocs/syst...m_en/techov.htm
It looks like it does not have an AGP slot so if you were to replace the video/graphics card it would have to be a pci type.
If your 3d aps used to run and no longer do you can stop some of the programs you have running in the background from starting when you boot the computer, but be very careful not to disable your firewall, anti-virus ap and anti-malware aps as well as anything Windows requires to operate.
What AV program are you using? Some use much more resources than others and your problem may well be be lack of enough RAM.
If the aps (games or other graphics intensive aps) you are using do not require being online, disconnect your computer from the internet and temporarily exit your AV ap. Stopping programs like Instant Messengers, shutting off your wallpaper, disabling active desktop if you are using that, etc, will help in that doing so will make more ram available to your system and the graphics aps you are trying to use.
Let us know if that helps.