I have a Western Digital 400 GB hard drive and use it for my PC operating on Vista. My friend used it for his new MacBook and his computer tried to back up on the hard drive. Everything got erased on it and now my PC won't recognize it. It shows up in the Disk Management but won't allow my to access it.

The data on the harddisk has been erased under Mac OS? In this case the harddisk was most likely formatted with Mac OS Extended filesystem. You need to reformat the harddisk again with NTFS to make it usable under Windows.