I want to run Computer virus on virtual machine to see how they behave, is there any online repository or database which store computer virus. Both old and new?
There are various research sites that keep malware samples available for research. I found a list of sites that provide downloadable samples here: http://zeltser.com/combating-malicious-software/malware-sample-sources.html
You may want to look at the EICAR antivirus test file.
You can create this file by opening a text editor and pasting the following into it:
The above text, in the words of the above site, is actually x86 machine language, and "... is a legitimate DOS program, and produces sensible results when run (it prints the message 'EICAR-STANDARD-ANTIVIRUS-TEST-FILE!')."