Have your checked you logging to see the real problem?
I suggest extracting the Mongo installation to c:\mongodb.
Create the c:\mongodb\logs and the c:\mongodb\data\db directories.
Then browse the the c:\mongodb\bin directory and run the following to remove the service (if you’ve installed it!):
mongod --remove
Then install the service, specifying the log and data directories:
mongod --logpath c:\mongodb\logs\mongo.log --dbpath c:\mongodb\data\db --directoryperdb --install
Then if there is a problem starting the service you should see the reason in the specified log file.
More info here.