Homegroup working, but cannot access desktop

Welcome to Seven Forums,

Did you give an 'Everyone' permission? See here how.

Also, try accessing the computer this way:
Click on Start, type \\ComputerName\C (if C Drive is shared off), you may also replace the computer name with the IP Address of the computer.

Hi morum, to me, Homegroup was a pain then I found a better way. Read this, it's easier than it first appears. File Sharing Settings.pdf

There is an NTFS Security feature from XP that's taken front row in Win7 called Ownership.
If you copy a folder or file from any other NTFS (Hard Disk Format for XP and up), it will have an OWNERSHIP property.
Fortunately the fix for this is easy.

I think if this is the problem, there's a quick shortcut to "Take Ownership" if you right click it.
Otherwise the long way around is . . .

Right Click on your Folder you want to access, go to
Properties, of the folder, go to the third tab,
Security Tab of properties, then the button at the botom,
Advanced Security, of security tab to open a new dialogue box, then to the third tab of that to,
Owner tab, of advanced security, and again a button at the bottom, called,
Edit ..., in advanced security where you can assign ownership.

Assign ownership to yourself. (There's a help link for that there.)
Hope that helps. :)