Passing a JSON argument to bitcoind in Windows command prompt
The important part is that strings should be enclosed in triple double-quotation marks. For example:

bitcoind sendmany "" {"""1ELvnrA6PhUyDBS6iR25K1Xx4xXL6VMfJX""":0.001,"""1NX3R5Le4c1y9E316NPMze7aEmCXjpCY58""":0.001}