First you'll need QuakeIII Arena installed on your system (I got it at a Wal-Mart for 10$).
1.Q3ToolSetup_Mar172000.exe
Install this file(.bsp build tools) to your Q3 directory.
2.GtkRadiantSetup-1.4.0-Q3RTCWET.exe
Install this file(map builder program) and set it up to use Q3Tools(instructions are in the help section).
3.(optional)GTKBuild_v20_full.exe
Install this file(separate build tool)and configure the same as GTKRadiant.
I use the third program because it compiles large maps much faster and is less prone to crashing.
Sorry for the late reply I had to upload these files on my free web service.
edit: Do some checking around at planetquake...you may only need the Q3 demo and not the full game(not sure about this).
Asus A7M266-D/1024MB PC2700DDR/2AMD MP1800+/Asus Ti4600 128MB