If you are using the built in video controller and haven't added any RAM, that would be your problem. It comes with 512 MB of RAM and the video controller shares it. The Dimension 2400 really wasn't built for games. Even maxing out the memory at 1 GB and adding a video card, the computer will basically suck while playing TS2, especially with Seasons.
You've got two options I think.
1) Buy another 512 MB of memory and a separate graphics card. (cheaper of the two options)
2) Buy another computer that is more of a gaming machine.
Your best bet is to print out a copy of the recommended specifications EA has for Seasons and for
your computer, go to your friendly computer store and ask the geeks there to help you either upgrade or replace your computer.