what you need to do is use the lcase and ucase functions as gyuan meantioned but you need to use them to locate the string and not replace it... once you have located the string then you can wrap it in bold tags,... some thing like.....
Code:
function highlight(yourString, strToHighlight)
tmpString = LCase(yourString)
startLoc = instr(1, lcase(yourString), strToHighlight)
endLoc - startLoc + len(strToHighlight)
resultString = left(yourString, startLoc)
resultString = resultString & "<b>" & strToHighlight & "</b>"
resultString = resultString & mid(yourString, endLoc)
highlight = resultString
end function
You should of course make this a lot prettier and robust and make sure that the start and end locations are right (they might be out by one) but in priniciple it is what you need to do