It could be the ACPI implementation on your motherboard. At the boot> prompt, enter "-c", which will allow you to access the User Kernel Configurator. At the UKC> prompt, enter:
disable acpi
quit
The boot_config(8) man page has more details.
If that solves your problem, make the change to /bsd by using config(8):
# config -euf /bsd
OpenBSD 4.3 (GENERIC) #698: Wed Mar 12 11:07:05 MDT 2008
deraadt@i386.openbsd.org:/usr/src/sys/arch/i386/compile/GENERIC
Processing history...
417 acpi0 disabled
Enter 'help' for information
ukc> quit
Saving modified kernel.
#