Debug Assertion/Loader/ Explorer Failures

I've been trying to remove a virus from my PC (Windows XP OS) and somehow messed something else up in the process. 

I think I've gotten the virus removed.  But now, after starting a PC session, I occasionally get the following pop-up:

Debug assertion failure  c:/windows/editpad.exe.  File: footprintf.c  Line 56  expression: str!=null

When I click on "retry" the window disappears and I can continue with the running program.  But it eventually comes back during a later session.

Then, usually the debug window is quickly followed by two others.  The first is a pop-up saying the "loader application has failed" and the second says the "explorer has failed." When I close these pop-ups, the PC usually seems to run OK.  But every once in awhile, the system reboots after I click on the "explorer failed" notice. 

I can't find any help with the Windows help files.  Any assistance from this forum would be appreciated.