I can't get the file sharing to work between my two computers, help please?

Ok, so I’ve got 2 computers: a Sony laptop and an HP desktop. Both run Windows 7. I just reinstalled Windows 7 on the desktop, and before I did that, I could share files.

But now!!! it no longer works.

I’ve set up a homegroup, created secondary user accounts on both computers (not guests), and enabled sharing of all important folders. But when I drag and drop something from one computer to the other, I get various error messages like “Destination Folder Access Denied – You need permission to preform this action” or “Network Error – Windows cannot access [folder path] – You do not have permission to access \\User-Vaio\C on [folder path]. Contact your network administrator to request access.

Now, it also doesn’t ask me to log in when I go to the other computer via My Network, which it used to, nor do I remember having set up a homegroup before.

Help please?