Hello! Avast sometimes detects a threat on the following page, and sometimes not (let’s it load normally):
www.viper-room.at
How is that possible? Is it possible that I got infected? Thanx! 
Dr. Web says it’s clean.
But I believe more in avast detections.
Generally, avast detection is accurate in these cases.
Isn’t it an encrypted/obfuscated script or iframe?
Wasn’t the site hacked?
Since you don’t say anything about the detection, mainly what the malware name was, it isn’t possible to say.
What is the infected file name, where was it found e.g. (C:\windows\system32\infected-file-name.xxx or URL in full) ?
Check the avast! Log Viewer (right click the avast ‘a’ icon), Warning section, this contains information on all avast detections. C:\Program Files\Alwil Software\Avast4\ashLogV.exe - Or check the source file using notepad C:\Program Files\Alwil Software\Avast4\DATA\log\Warning.log
Sites are getting hacked with monotonous regularity and avast is very hot on this relatively new exploit, so it is possible that a site gets hacked but also gets cleaned up by the owners. If they don’t do a good job of cleaning up or more importantly closing the vulnerability that was exploited to infect the site, then it is very possible that it could happen again.