Did you start your installation with Diablo2:LoD v1.10 (perhaps via
Ensley03's File Tree, which comes with D2SE installed). If you started with a fresh download of D2 from Blizzard (LoDv1.14), then that is not backward compatible with many D2 mods (LoDv1.10) or D2SE (a mod manager awesome for smoothing out issues with modern operating systems).
Which operating system are you using? If you're using Windows, did you install into the ProgramFiles folder (modern WinOS permissions make it worthwhile to install D2 outside of that folder, e.g. C:/Diablo2/ rather than C:/ProgramFiles/Diablo2/)
Are you using a powerful desktop computer, or a Netbook (like me, I use DirectDraw video mode because my machine doesn't seem to handle Glide well at all)?
Did you run the VideoTest? Which video modes have you tried (generally DirectDraw and Glide are the ways to go)? If you're using Glide, are you using the Glide wrapper (It happens to come with D2SE if you're using it).
The Glide Wrapper allows you to play windowed mode with double size, and is generally a good idea for modern video cards and operating systems (but not guaranteed for everyone).