First, power the computer down completely; leave it off for five or ten minutes, then start it again. See if that's cured the problem. If not ...
I'd check to see if there are any add-ins in PowerPoint or other Windows programs that use ESC for special purposes or trap and disable it deliberately.
If you have diagnostic software, it may include a keyboard test; try that to see if the ESC key is working consistently. If you don't have a program to test with, this might help:
http://www.passmark.com/products/keytest.htm
(I haven't tried it myself, but I'm pretty sure passmark is reputable)