My avast home antivirus detected some win95 and win98
viruses on my windows XP. Internet informations show me that those viruses exist no more…

Your info is wrong. Viruses do not disapear. There is always someone who still have them.

If the name of a virus has win95 or win98 in it, it doesn’t mean that they only work on that particulair OS.

I suggest you start with running a boottime scan and let Avast do its job. After that run Ad-Aware and Spybot S&D.