So I came downstairs this morning to my computer and low and behold a message was being displayed that says 'Error loading OS' in white lettering on a black screen.

I don't know what happened because my computer has been running for the past several days in Windows. I never  logged out or shut down.

Any ideas how to fix this? Losing my data on the hard drive to reformat is really not an option.

You're saying "has been running for the past severeal days in Windows"

Is this a Mac with a dual boot thing?  If that's the case, your boot block is screwed up.  You gotta ask some Mac people for help with that.  If it's just a plain Windows machine.  Do what Kolop said.  But don't do it if it's a Mac or you won't be able to get back to OSX.

It's likely it's your master book record.  What you are seeing is a BIOS message that says it can't find your Operating System.  This typically happens when the boot block gets corrupted, but these-days it's less likely to happen unless you have a virus or drive failure. 

If you do manage to restore it, be sure to back-up any important files as it might be a precursor to a more serious problem with your drive.

Well I pretty much tried everything last night to no avail. I'm still not able to access the HD, so pretty sure I lost everything.

I booted up with the Windows cd and did a repair. I also tried the fixmbr and fixboot commands.  The weird thing is the BIOS recognizes the hard drive is still there, and it shows when booting up, but it can't find the master drive so it assigns my slave hard drive as C:/. 

I ended up installing Windows XP on my second hard drive and booting into windows.  Upon loading windows it ran a fix on my messed up hard drive by finding the bad partitions, indexing, etc... After that it loaded up the OS and I went into My Computer. Both HD's (good and bad) showed up. I clicked on the bad hd to access it and received the following error:

is not accessible the request can not be performed cause of a I/O device error

Lol.  Hopefully I learn from this and back all my crap up from now on. Really sucks.  Do you know how Apple is on allowing you to retrieve all your iTunes library from them?