Why?

[eminence]

When I use several software ,the AVAST! anti-virus prompt that there has trojan?But those software are no issue.

[Lisandro]

To know if a file is a false positive, please submit it to JOTTI or VirusTotal and let us know the result. If it is indeed a false positive, send it in a password protected zip to virus@avast.com
Please, mention in the body of the message why you think it is a false positive and the password used. Thanks.

As a workaround, you can add these files to the Standard Shield provider (on-access scanning) exclusion list.
Left click the 'a' blue icon, click on the provider icon at left and then Customize. Go to Advanced tab and click on Add button...
You can use wildcards like * and ?. But be carefull, you should 'exclude' that many files that let your system in danger.
After that, please, periodically check it - scan it into Chest, right clicking the file -  there should still be a copy in the chest even though you restored it to the original location. When it is no longer detected as being infected then you can also remove it from the Exclusion list.

This link is a tutorial on how to help correct a virus detection that you believe to be false:
http://forum.avast.com/index.php?topic=25009.msg204838#msg204838

[DavidR]

What are the malware names, file names of the infected/suspect file, where were they found e.g. (malware name, C:\windows\system32\infected-file-name.xxx) ?  Check the avast! Log Viewer (right click the avast icon), Warning section, this contains information on all avast detections.

As Tech suggest you need to confirm that the detection is correct or otherwise bay scanning at VT or Jotti.