Bob has a 64GB hard drive which isn't very big and fills up fast, especially since Windows will take up about half of that. He should first find out what is on his hard drive and clear out a bunch of things that he doesn't need. To do this, Leo recommends a program called WindDirStat. On the Mac there's a program called DiscInventoryX. This gives a visual display of what's on the drive. He can search in Windows by size, so he could find big files without downloading anything. He should be careful of what he deletes, because you don't want to get rid of vital system files.
The other thing he could do is backup anything he doesn't need. He can get a 1TB external drive for under $100. He could backup his data, and then start over by reinstalling Windows. He also could look at all the programs he has installed in Control Panel and uninstall things he doesn't use.
He also can offload a lot of files into the cloud. Google Music or Amazon's cloud music service would allow him to keep his music in the cloud. The other advantage of doing this is that his music would be available on all of his devices. Microsoft SkyDrive will give him 7GB of offsite storage in the cloud.