Your best bet is probably going to be to uninstall and reinstall your video drivers. This may involve removing the device and associated software via both Add/Remove Programs (if there's an NVidia entry present) and Device Manager. It would also be a good idea to check for updated drivers from Dell (assuming that the video card came with the system), although it appears that their most recent offering is dated February of 2001. Also, Windows Update frequently shows updated NVidia drivers for those systems, so that's another potential source. I've used the Microsoft drivers on a number of occasions, and have never had any problems with them.
http://support.dell.com/support/topics/global.aspx/support/kb/en/document?DN=RG1074200 How Do I Install the NVIDIA 32MB TNT II M64 4X AGP Video Driver in the Microsoft® Windows® Millennium Edition (Me) Operating System on My Dell™ Dimension™ 4100?