I have a Dell Dimension 4550 with a strange boot problem.  I have made
no changes to my hardware, software or OS.  I am running Windows XP
Pro.  On my last boot, I received the blue screen of death with the
following message: "PAGE_FAULT_IN_NONPAGED_AREA".  In the technical
information area it says: "STOP: 0x00000050 (0xF7C3A000, 0x00000001,
0x804DA232,0x00000000)".  It then begins a dump of memory.  When
complete the whole process repeats - forever!  I suspect I have been
infected even though I run AV software and keep it current (daily
checks).  I then attempted a safe boot by hitting the F8 key.  Every
time I attempt this the screen goes black and nothing else ever
happens.  I then attempted to boot from CD or Floppy by entering the
BIOS setup to change the boot order.  When I hit the F2 key to enter
setup, I receive the message "Entering Setup" and then the screen goes
black and nothing else ever happens.  I have changed keyboard, mouse
and monitor with the same results.  I have reseated memory and all
cards with the same results.  What is happening?  I can I repair my OS
and get back to business?

I had the same issue in the past with a Dell Latitude. In my case it
was a memory error based on a bad motherboard. Actually I've replaced
the motherboard 3 times during the 3 year warranty for the exact same
issue. The memory on Dell (and most other laptops) has no parity and
won't be able to give you a memory error - it just crashes.
Try to make sure the laptop is on a solid desk (not on your knees or
anything else shaking) and boot the diagnostic cd (I hope you got it
with the laptop) and run the hardware tests. If you crash before you
are able to boot the diagnostic cd, then try to swap the memory simms
(if you have 2)
If you don't have the diag cd, then try to replace the 1st memory simm
(if you have 2 memory simms, then remove the 1st and put the 2nd in
it's place).

There are many other reasons for a crash, but if you crash jsut trying
to go into setup (by pressing F2), then this is most likely not a
software issue.

What you can do depends on how far in the boot process you can get. 
You have the option to use Microsoft's Recovery Console, this link is
helpful:
http://support.microsoft.com/default.aspx?scid=kb;en-us;307654

If you think it's a virus you can take the hard drive out of the 4550
and connect it as the second drive in another system that has a
current updated AV loaded.  Then scan the second drive for viruses.

While you have drive attached to the other system... pull your
important files off, if you dont have a current backup.

Page faults are caused by failed RAM or a hard drive with a bad block.
Test the RAM by creating a memory test disk from www.memtest.org and
you'll know within minutes if you have bad RAM. Next, determine the
hard drive manufacturer and go to their web-site. Create a diagnostic
diskette and test the hard drive.