Whats problem with her computer and how to fix it?
She can hear my sound, but I cannot hear her sound.

she also called to other friend and other cant hear her.
Usually when she calling to me, I canot hear her.
So I just hang-up call, and calling back. (then I can hear sound)
but sometime even that wont work.

I use windows 7, she use windows vista.

It sounds like the system she is on has problem detecting her microphone or microphone settings.
What microphone is she using? Maybe she has disabled it.
You could also ask her to check Control Panel - > Sound during calls to see if her recording device is flagged as disabled while talking.

Also make sure Skype has the right recording device recognized.
In Skype: Tools -> Options -> Audio Settings -> Confirm the Microphone and the sound volume on it.
Make sure it is not set to automatically adjust the sound volume.

Tooked option off "set to automatically adjust"
microphone on build in laptop. its working just sometime I cannot hear anything. maybe this solving problem. I dont know, I contact back here if problem continues.

Just gonna throw this out there coz my sister had issues last year with this, is her laptop Muted? Or volume all the way down? That was my sisters problem lol

Retry the soundtest. Otherwise the problem may be on her end.

Volumes are max.
and even before intalled SKYPE, we called via Facebook videochat. there was same problem. I think problem is not with the skype.

sound test works perfectly on me.

Running skype in compality mode: "Windows xp sp3" fixed all issues (it will now also minimize to system tray)
and I changed automate volume control to "Off"

Topic can close now. and for small volume audition I downloaded dfx volume booster, that free version works fine.