From what you sat, no activity from avast icon, etc. I doubt that it is that. Your system, PII 400mhz would be considered a little underpowered, that combined with win98, you could well be running out of system resources (not particullarly well handled by win98).

Try and reduce the number of programs running, unless you really need them.

The best way to examine what is running on your system is to run hijackthis and post a copy (cut and paste) of the hijackthis.log here.

Download HijackThis.zip, create a folder 'HiJackThis (or any name you choose) and unzip the contents into it. Run the hijackthis.exe and scan and save the log.