What I would do is first rank you records from 1 to whatever (10) then I would divide the number of records (count *) by 5 and depending on the ranking assigne and where it falls into place (which fifth) assign it an A,B,C,D,E.
In the ms knowledgebase look for a file called "QrySmp00.exe" it has a sample DB that has several sample queries. One will show you how to rank you records. as far as the other parts, you are one you own. It is pssible to do what you are after, but it may take more then one query to do it.
Depending of you skills with SQL and VBA, it may be faster to write a VBA code that does this for you and write the quintile back to the orginal table.