This issue can happen after installation.

Right-click on the PowerLink icon and select to Run As - Administrator. (Note: if you normally launch PowerLink from Attache you should still have an icon). This should get you to the login screen. After this, close PowerLink and run again as normal.

If this doesn't resolve the issue, you may need to reinstall Powerlink.

Notes for Windows 7 / Vista users:
On a Vista machine you may need to give the user full administration rights and then right click and run as administrator.