Silly question ......have u replaced the ping.exe file...it may be corrupted...
Do other files run ok from the command prompt?
Try copying the file to the root dirctory and running..ie c:\
strange one this.....
Ok....after windows and dos boots , when an application is called from the command prompt dos will look only in the root dirctory and windows(or system).......if ping is in neither of these it wont be found......further dirctories can be defined in msdos.sys in the root folder,or put ping.exe where it can be found (simpler)...its in windows on mine which is also why command.com is in the same places.
I'm not a dos expert but if you google for info on windows and dos there some helpful info out there.