problem with MSVCp60.dll

Hi folks, this is my first post so be gentle .
Running Win98 SE.
I'm visiting my bro and I saw he was using his IE browser. Wanting to do him a favor I installed Firefox 2.1 and told him he should only use it instead of IE for obvious reasons. Everything was fine untill my niece tried using her MSN Messenger she would get an 'Illegal operation MSVCRT.DLL corrupted" or some other wording(forgot to write it down),but It was the MSVCRT that was in error. CouId not use Firefox or IE without getting same error. I then uninstalled Firefox 2.1 and installed Firefox 1.5 with same result.I restored the original MSVCRT.DLL but got same errors.I uninstalled Firefox 1.5. Everything was ok untill my Bro tried to access his Active Worlds(online virtual community) he got 'DLL loading error MSVCP60.DLL' Tried restoring original MSVCP60, but got same error.Tried newer version of said DLL,got same error. Can anyone help? Sorry for being so winded but being an ex system programmer I know
it's important to list everything that happened before error the problem occured.