Do you have all Windows updates and updates to your system and any hardware devices by the manufacturers?
Please see these:
Also, you can use the Skype client program window at anytime and do:
Hellp -> Call Quality Guide -> Sound Check Wizard
Note: The Sound Check Wizard also supports Video as well.
A new version has just been released September 1, 2011:
Please install over what you have using this and when prompted, please choose open or run: http://download.skype.com/SkypeSetup.exe
This new version is 18.104.22.168 to check what version you have, using the main Skype client window please do:
Help -> About Skype