Network / Sharing :: Restarted XP Computer Now Cannot Access File On Windows 8

I had to restart a computer on our office network which uses Windows XP on Wednesday, September 17, 2014. Now I cannot access a file on a computer which uses Windows 8 unless I tell the Windows 8 computer to let EVERYONE access the file - which I don't want to do. A computer using Windows Vista on the same network has had no problems. I've created a new username on the Windows 8 computer and given the problem computer that name but still no access. I've enabled read/write file sharing and given the problem computer full access permissions but still can't get in. I removed all Microsoft updates that had been installed four days before the problem occurred on the Windows 8 computer and recovered the Windows XP system to the day before the problem occurred - still can't get in.