We might need a little more info but since you just added a new HDD, could it be that it's it's a very large drive and larger than 137GB? If so you may need something like Maxtor's Big Drive Enabler, at:
http://www.maxtor.com/en/support/dow...ve_enabler.htm
Check out the KB articles it links to on that page too.
Memtest86
http://www.memtest86.com/memt29.zip
will make a boot floppy that tests your memory (you shouldn't see any errors), but normally if you have bad RAM the whole thing crashes, not just Explorer....