The HOSTS file has been set to read only or locked by another process.
Some types of Spyware software or AV software may set the file attributes of the windows HOSTS file to read-only and the Signup client needs to access this.
Most common is SpyBoy Search & Destroy. Please check the web site for your specific spyware application and search for HOSTS file to discover further on how to insure that the file is not reset to read-only upon next reboot.
Alternatively you can locate the windows HOSTS file at:
Right-click on the file and deselect the checkbox for read-only.
Re-install the Signup client