Here's another way to do it which handles larger files - though there is a leading comma on each line.
@echo off
for /f "tokens=1,* delims=: " %%a in (file.txt) do (
set /p "=,%%a">>filea.tmp <nul
set /p "=,%%b">>fileb.tmp <nul
)
echo.>>filea.tmp
copy /b filea.tmp+fileb.tmp file.csv
del filea.tmp
del fileb.tmp