On second thoughts, I'm not sure it can be done in Excel unless each word of your sentences is in a separate cell of a spreadsheet i.e. each sentence is on a single row, with each word in a single cell on that row and no punctuation.
I'm not even sure that you could do it the way I outlined in Access either, come to that.
There is a way to do it using Excel and the word counter and text analyser at
http://sporkforge.com/text/word_count.php
If you go to that page, scroll down and click on the browse button under the text entry box and select the file with your sentences in, then click on the Upload File & Analyze Text button, it will, among other things, return a list of the words in your file with a word count for each of them. You can copy that list and paste it into Notepad, then load that file into Excel along with the your individual words list, and then use Vlookup.
OK, so it's not quite as simple as that and you will, perhaps, have to do a bit of tweaking of the Notepad list, but it should work and be a lot faster than trying to find ways to do it exclusively in Excel. The only limit with the Sporkforge link is that your file containing the sentences has to be smaller than 5Mb.