I bought a Dell Pentium 4 with Windows XP a couple of years ago and
installed a floppy disk drive.  I never had a problem using the floppy
drive before.  Now, I cannot access the floppy drive.  When I try to
open a file from a disk, the computer does not give me the A: drive as
an option any more.  I have tried opening from Windows Explorer and
various programs.  I have also tried more than 1 disk to eliminate the
disk as the problem.  Also, when I run a virus scan, the A: drive is
no longer one of the choices of drive to scan.

The only tip in the Dell user manual is that a virus may interfere
with the computer recognizing the drive.  I have scanned and cleaned
with the latest version of McAfee and I'm virus free, so I don't
believe that's it.  

I have gone into the setup screen (F2 during
startup) and I see that it lists a 1.44 MB Floppy Disk Drive as 1 one
of the drives.

* What can I do to get my A: drive back again? *

As a last resort, I can open the PC and check the physical connection,
but I do not believe that could be the problem since the PC has not
been physically moved sinced installation.  I want to try
"programming" fixes or tests first.

I will reply promptly to requests for clarification.

This doesn't really qualify as an answer, but I had a similar problem
a few years ago, which *did* turn out to be hardware-related, even
though the machine had not been physically moved.  I had a cable fail
on me, which caused Windows not tbe able to read the presence of the
drive, even though BIOS (Setup) registered it as being there.  You
might want to look at hardware-related issues, if for no other reason
than a floppy drive is probably the single least expensive part on a
PC that can fail :-)

To check if your floppy really works, make a boot floppy on another
system, and insert it into your own (check BIOS that Floppy is the
first boot device). If the floppy boots (one formatted on another XP
with "Create an MS-DOS startup disk" option checked), it means the
hardware is okay, and the easiest way it too reinstall XP (if there's
no way you can get a bootable floppy, you can also try a linux CD boot
(knoppix or mepis) and read/write to floppy from there). You can also
try to install with the Repair option (boot from CD, with for it to
stop, press enter, then F8, and see if it detects the previous XP, and
it will give you an R (epair) option). If not, reinstall XP cleanly to
solve the problem (you'd have to reinstall all your programs and
data). If repair is possible, then the error in the system could still
carry over, and you'd have to do the clean reinstall anyway.

Most likely, it is caused by a hardware failure due to the floppy
drive not working. I would remove that drive and try it on another
computer to isolate the problem. If the floppy drive does not work, I
would say that the floppy drive is not functioning. You can get a new
floppy for less than $8
i.e. http://www.directron.com/cd---storage-floppy-drives.html

If the floppy drive works in another machine, then check all your
cable that goes from the motherboard to the floppy drive. Ensure that
all of the cables are securely inserted in the right ports, from the
power cable to the IDE cables.

If checking all the cables did not fix the problem, then there is
usually a software that might have cause the conflict or disable the
permission to access the floppy drive. Check that to ensure that the
floppy drive has all the right permissions. Also, run a "Add Hardware
Scan" to locate that hardware.

I would replace the floppy if it is not really necessary.
With USB storage drive being so cheap, just use that as opposed to floppy.
A 128MB USB storage pen drives cost less than $20 which is more
reliable than a floppy. http://www.directron.com/usb128mb.html

goto start>control panel. You may need to switch to classic view and
click add hardware if it doesn't automatically detect it find it on
the list that worked for my friend who had the same problem.