Ok there are a few possible solutions, lets start here..
On the Tools menu in Windows Explorer, click Folder Options.
Click the View tab.
Under Hidden files and folders, click Show hidden files and folders.
Note To access Windows Explorer, click Start, point to All Programs, and then click Windows Explorer.
Now that that is done, start a search on the computer, by clicking start, and search..
type this in or copy and paste..
if you find that file change it to System32/inf/wdma_usb.inf
Now, try to adjust the microphone volume in call central.. will it let you increase it now?
If not, lets try right clicking on the volume icon near the clock on the lower right hand side of your taskbar..
Rightclick, choose recording devices, find the logitech headset listed, click on it, click properties, click levels, and see if it will let you turn the volume level up to 100, and you may even want to increase the microphone boost level..
Is the mike now louder when using skype?