Sounds as if the computer graphics aren't man enough for the job. Have you tried to upgrade the video drivers?
The best option would be to add a video card. Unfortunately this is where I find conflicting information, one web site says that the Dell Dimension 3100 motherboard comes with only on board graphics and does not have either an AGP or PCI-e slot,and another web site says it has a PCI-e slot free.
Open it up and have a look, if there's a spare PCI-e slot (Google images to see what one looks like) buy a graphics card with 256/512 Mb of RAM and fit it - that will solve your problem.
If however it doesn't have a PCI-e slot, the only real solution is to buy a new pc.