As has been said before, you need to run SRB2 with the command line parameter -opengl.
If you run SRB2 using a desktop shortcut, you can place the parameter after the quoted address in the target field.
"C:\Documents and Settings\randomfolder\SRB2\SRB2 2.0\srb2win.exe" -opengl
Like that. Don't change the settings in the quote, just pay attention to the bold.
If you don't, or would rather make a .bat file, just open Notepad, make a file that says
srb2win.exe -opengl
and then open the Save dialog box, go into the same folder SRB2 sits in and save it as a .bat file. This means you do not set it to a .txt file in the save as type section. You set it to All Files and manually add the .bat extension. It goes after the filename, no spaces. (Not that spaces would break it, they'll just look silly)
If this explanation flies over your head, I'll provide screenshots, or someone else will.
Also, make sure you use the latest SRB2 version (2.0.6)
A lot of people did/used to get this wrong.