several methods:

with monkey's audio frontend
a. Get lame.exe from here.
b. copy lame.exe to the monkey's audio folder.
c. drop your ape files to the frontend
d. select compress -> external encoder -> MP3

with foobar2000:
a. install foobar. Get it here.
b. Get foo_input_monkey from here. Extract the zip file, copy the foo_input_monkey dll to the "components" folder in your foobar directory.
c. Get lame.exe from here. Copy the lame.exe file to your foobar2000 directory
d. Now open up foobar. Drop all the APE files you want to convert. Select them, right click them and select "Convert to" -> "convert to same directory". Choose MP3 as your encoder, choose your desired quality setting.

with Dbpoweramp:
a. Get dbpoweramp from here.
b. Install the monkey's audio codec for dbpoweramp from here.
c. In windows explorer, select your ape files, right click them, click convert to. Choose MP3, select your settings.