on the page http://docs.mongodb.org/manual/tutorial/install-mongodb-on-windows/
you say:

C:\>echo logpath=C:\mongodb\log > C:\mongodb\mongod.cfg

but when you execute the next command, it fails:

C:\>C:\mongodb\bin\mongod.exe --config C:\mongodb\mongod.cfg --install
all output going to: C:\mongodb\log
logpath [C:\mongodb\log] should be a file name not a directory

So I did this instead:

C:\>echo logpath=C:\mongodb\log\mongo.log > C:\mongodb\mongod.cfg

and it works for me now.