Freezing probs

Hi everyone!
I have two "freezing" issues with Windows98:
-first one happens when closing Internet Explorer: the system does not respond for about a minute, then comes back to life.
-second one is more serious: sometimes, when the system is searching files (aka trying to display a window, or even using Word or Winamp when I try and load files), it just waits for a while (10 secs) then freezes completely (mouse doesn't move, ctr-alt-del doesn't work), and I have to reboot.
What do you reckon this could be?
PS: windows version updated, and disks defragmented and scandisked (and I am pretty confident about spywares, adwares and viruses...)
Thanks!