It is most certainly suspicious as a) autorun.exe is a strange file to have in the root folder, b) there may also be an autorun.inf in the same location and that may run this file and c) there are some adverse google hits for this file name.
http://www.file.net/process/autorun.exe.htmlIf autorun.exe is located in C:\ then the security rating is 22% dangerous. File size is 508555 bytes (33% of all occurrence), 303104 bytes, 327680 bytes. The program has a visible window. autorun.exe is not a Windows system file. There is no description of the program. autorun.exe is able to record inputs.
http://www.threatexpert.com/files/autorun.exe.htmlYou should submit that file to avast so it can be further analysed.
I would also suggest you rename it as say autoSUSrun.exe so if there is a command in the registry or an autorun.inf file to run autorun.exe then it won't find that file (as you renamed it).
You could also check the offending/suspect file at:
VirusTotal - Multi engine on-line virus scanner and
report the findings here the URL in the Address bar of the VT results page.
If you haven't already got this software (freeware), download, install, update and run it,
preferably in safe mode and report the findings (it should product a log file).