Ok, here's some more information:
My pc is an Athlon64 3000+, 512MB DDR, 60GB HD, Radeon 9550 (256MB)
Operating system is Windows XP SP2 with all updates installed
What happens is that when I plug the cable of my phone in and go to explorer, normally a pictogram with Data on it appears quite fast, but now it takes a lot longer. Then when I click on Data it seems to take forever before the directories appear. When it finally appears and I click on a directory, explorer is not reacting anymore. ashServ.exe takes about 60-70% cpu-usage
(normally 0%). Then I stop Avast's standard shield and immediately everything is fine again, explorer reacts again and I can browse my data.
Hope this is enough information