how come i have a virus on my computer and avast can’t find it when i scan the computer? 
No antivirus solution will have 100% detection. It’s just not possible, because malware writers will always be one step ahead of antivirus vendors. I recommend that you use the avast! sandbox included in the Pro version to virtualize your browser for safer web surfing. (Right click the application and select ‘Run virtualized’ or ‘Always run in sandbox’)
Have you detected/identified the virus by some other means?
First, no single security application is going to give 100% protection.
Second, how do you know and or what detected it ?
Lastly, what is the virus name, the file name and its location ?