Make a new notepad document and save it as a .bat file with the following text: "srb2win.exe -win"
By double clicking your new batch file, SRB2 will start up in windowed mode.
Also, if you still have it, it's possible to use the SRB2 Net Launcher from the 1.094 days with SRB2 2.0 except a few options (OpenGL) won't work.