Few things. On windows Vista and up you have to change some Home Group Options. Go to the Network and Sharing Center.
Then in the Change your networking settings click on Choose Homegroup and sharing options. Then click on Change Advanced sharing settings.
make sure you are on Home or Work - and make sure Network Discovery, File sharing are turned on and turn off Password Protected Sharing.
Then on the drive you want to share you have to 1) make sure the Security Tab on the Properties page make sure Everyone is allowed to read and/or modify the drive IF you don't want to use a username and password to access it. and then in the sharing options under the Permission button make sure Everyone is in there as well.
If you have two PC's and they username and passwords are the same there shouldn't be an issue. But if you want to use a username and password you either have to 1) use the existing user name or 2) create new ones on the share PC for that.