Isass.exe entry point not found?

i have a MAC on which i run Parallel Desktop with Windows XP. Recently i have been having problems. the following message comes up. Isass.exe entry point not found. the procedure entry point could not be located in the dynamic link library RPCRT4.dll
i don't seem to be able to do anything. can someone help me know what has happened and what i can do?
thanks.