You've done an excellent job of troubleshooting so far and it really sounds like a hardware problem but I understand the thought of going through the trouble (not to mention the expense) of a keyboard replacement only to find that it does not fix the problem
Physically testing the keyboard with common electrical test equipment would be more difficult than replacing it. Here in my shop, I'd try it in a similar if not the same model laptop to see if it still has problems with the 'c' key but I'm guessing that you don't have two of these. We're getting into the realm of removing the keyboard anyway so why not just replace it if you're going that far.
Here is what I think may be the last thing left to test whether or not it is the keyboard without removing it physically.
Get a copy of xPUD. Boot to it from a CD or USB key and you will be running a completely new operating system. See if you still have problems with the 'c' key and if you do, it is probably a physical problem that requires a replacement.
You can get xPUD at
Basically, it has you burn an image to a cd and then boot the computer from that CD
It tells you how to get UNebootin and use that to put the image on a USB key and then you boot the computer from that USB key.
(you do not want to run it from Windows or in a virtual machine for your keyboard test)
If you need more help with this let us know.
Once you boot up your computer using xPUD, I would test the 'c' key like this...
click Terminal Emulator
Hit enter and start typing words with the letter 'c' in them to test it.
when done press
Then you can play with xPUD or power down the laptop, remove the CD or USB key and boot it back into Windows. Hopefully, you've learned more about whether your letter 'c' issue is hardware or software.
I notice that you list your OS as XP SP2. Why not SP3?
If you suspect an infection, it would be best for you to ignore my all of suggestions and head straight to the Am I Infected forum at
and ask there for help confirming whether or not your system is clean.
Hope some of this helps