PC hangs/freezes

I have a customer who has tried to instal new memory in their HP Pavillion 6355 then was unable to get Win98 to boot up. I have changed the chips (2x64mb)with known good ones, checked all connectors etc, backed up hard drive to spare, reformatted hard drive (twice), fresh instal of Win98, with only motherboard, hard,floppy and CD drives installed, cleared CMOS. Windows now starts up ok but entering into any program causes that program to hang/freeze and I have to use task manager to get out. Also Windows will not close down or restart (just hangs again)unless I am in safe mode when it closes OK. What should I check next? It has a ‘SPAX’ motherboard, AMD-K6 333 processor and 128mb RAM.