windows variables

Problem:
Windows 7 Ultimate 64 bit

All of a sudden , in order to run batch command such as taskkill, I need to do the following:
c:\windows\system32\taskkill

Before I just need to use the line taskkill and not define the full path to the exe file

I assume this is a system variables issue but I cannot see where the problem is
I have tried adding the "c:\windows" and "c:\windows\system32" to my path varuiables under Path but I am unable to append any new text to the Path which is vvery strange in itself
%SystemRoot% and %SystemRoot%\system32 are in the Path text already

Help appreciated please