Scan your computer and see if there's any virus on it
http://housecall.trendmicro.com/

or check for trojan or malwares, or spyware
http://www.safer-networking.org/

see if your computer and os can satisfy the requirements of skype:

• System requirements
• PC running Windows 2000, XP or Vista. (Windows 2000 users require DirectX 9.0 for video calls).
• Internet connection – broadband is best (GPRS is not supported for voice calls).
• Speakers and microphone – built-in or separate.
• For voice and video calls we recommend a computer with at least a 1GHz processor, 256 MB RAM and of course a webcam.
• ForHigh Quality Video calls you will need a high quality video webcam andsoftware, a dual-core processor computer and a fast broadbandconnection (384 kbps).