Well xrun.exe isn't an avast file and isn't on my system.
This sounds like a poece of malware that has previously been removed, but there is either a startup entry or other registry entry to run it. Because the file isn't there to run you get a windows error.
A google search for xrun.exe returns many hits,
http://www.google.com/search?q=xrun.exe.
If using winXP or Vista
SUPERantispyware can detect this and should be able to get rid of the registry remnants.
If the detection is by the web shield it is blocked from being downloaded to your hard disk so my be why you don't find the file.
