If you are using VC++6, download the tool : prjconverter (search at google and you'll find it)

Copy that in your Quake 3 code sub-directory.

Open up a command window. Change directory to Quake 3 code sub-directory and type.

prjconverter quake3.sln

It should make some project files.

Now load up Quake 3 code in VC++. Click on Projects --> Active Configurations. Change it to Win32-Release.

Unload the UI Project, it can create problems while compiling. Make sure that the Quake 3 project has botlib and renderer as its depedencies.