compaq presario 4800-pII-266-96mb sdram

on a customers pc system hangs at boot up after depassing the pass. we see only compag-sign.
i tested drive with antivirus-cd(2002). no virus.
can i execute the following commands ny using a standard boot-disk?
fdisk /mbr
sys c:

if i can enter with safe mode, what can i do?

maybe sdram (64+ 32, pc-100) should be replace. i will run test-program. do you know an alternative to ctram51? sorry i have the link somewhere, it was posted here.
maybe i will clean the contacts before executing the test.

i do not know yet how to enter the bios. i do not see anything at start. i will check compaq.