bassbag,
I use Spybot with Avast Home in one of my machines running Win98 without a problem at all.
System freeze can occur for a few reasons: lack of resources, program conflict, or hard drive errors.
What I would recommend is to reduce the 'custom' level of the scanner and try again.
Do you have any other antivirus software installed?
Did you ever have Norton installed?
When the system freezes.....for how long?
Does it require reboot to free it?
thanks
techie101