My home PC has Avast 4.8 and the PC at the school, where I am a teacher, runs Avast 5. Each time when I use my memory stick in the school PC and later use it at home, the Avast 4.8 reports the autorun virus/worm. That means Avast 5 does not detect the virus???
Welcome to the forums, dcronje 
Have you run a boot time scan on the computer that has avast 5?
If not, try that first.
No I am sure avast 5 detect that but you may try to open or click on the virus in the system which have 4.8,while the other PC which have avast 5 you may dont open or try to run or automatically run the virus.In a simple way:
the 4.8 PC may have autorun option enabled that mean not well protected so the system try to run the autorun and avast block it.
the pc with avast 5 maybe have autorun disabled so the malware dont run,To be more sure do scan for the memory stick.
I wish I help
does the school computer have another antivirus program besides avast?
Can you isolate the offending file (i.e., the one that is being automatically run) and send it to www.virustotal.com?
It could happen as an error on version 4.8 (false positive). Why don’t you upgrade to version 5?
Sorry, I didn’t visit the forum for a while.
The PC at the school does not run another anti-virus program.
What happens is this: there is another PC with no antivirus program which is infected with the AutoRun-gen2 virus. After I 've used a memory stick on that PC, and then put it into the PC with Avast 5, nothing happens. However, when I put it into the PC running Avast 4.8, it immediately reports the virus. If I delete it on my memory stick, it gets infected again when I use it on the PC with Avast 5.
Have you tried uploading the file to www.virustotal.com
could be a FP.
I did try, but I cannot get hold of the autorun.inf. However, I noticed something interesting; when the warning appears for the first time, and I do nothing, but pull out the stick and then put it in again, there is no warning.
Autorun Eater 2.5 http://www.softpedia.com/get/Security/Secure-cleaning/Autorun-Eater.shtml
Flash Disinfector – free autorun.inf trojans removal tool http://www.myantispyware.com/2009/01/08/flash-disinfector-free-autoruninf-trojans-removal-tool/
Panda USB Vaccine - Antimalware and Vaccine for USB devices http://www.pandasecurity.com/homeusers/downloads/usbvaccine/