Thanks for giving me a hand ^^
Old antivrius on that computer was Norton antivirus. However, no need to uninstall in order to get rid of it, in my case. I recently had to format PC's HDD to fix a hardware problem and all data on it is gone (nothing critical). I next began to reinstall stuff and applications, and thought it was my chance to test Avast. So I downloaded and installed it as well. It worked fine for a weak. And this morning I got the message mentioned above.
So, in your opinion, an Xp SP3 update would solve the problem?
Weird...
To tell the truth, I was avoiding this update on my own, knowing that Xp SP3 needs more ressources than SP2.
But if it's the only way to solve this...