Riched32.DLL error--windows xpp software

Since August I have been getting the following message "the application or DLL.C:\windows\system32\riched32.dll is not a valid windows image Please check this against your installation diskett"  It is affecting some of my programs from operating properly.  Windows will shut down with no apparent reason.  I have had this computer for over three and a half years.  So  I  can not just reinstall the  cd microsoft windows xp home edition including pac 1 because it does not include pack 2.  I need help.