which Video codecs I have?

Use this highly use ful tool....
*www.snabbstart.com/download/gspot-codec-information-appliance.aspx

If you want to know which codecs are installed than either use a third party software like Everest or goto
Start>All Programs>Accesories>System Tools>System Information
In the System information go here
Components>Multimedia
Here you will get all the codecs installed.

No there is no problem having multiple video codecs on your system.

Is there any problem to have more than one video codecs installed ?

because I tried to install Kazaa codec pack,it installed properly ,But after that I can't open Quicktime Player ,it was giving an error called it is corrupted,And the system hangs up.

Try using K-Lite codec pack available on Digit CD. It gives no problems. With Kazaa even i had some problems