=CHAR(CHOOSE(RANDBETWEEN(1,2),RANDBETWEEN(48,57),R ANDBETWEEN(65,90)))
to get a random alphanumeric character... or, 5 of those concatenated to get a 5 character one: =CHAR(CHOOSE(RANDBETWEEN(1,2),RANDBETWEEN(48,57),R ANDBETWEEN(65,90)))&CHAR(CHOOSE(RANDBETWEEN(1,2),R ANDBETWEEN(48,57),RANDBETWEEN(65,90)))&CHAR(CHOOSE (RANDBETWEEN(1,2),RANDBETWEEN(48,57),RANDBETWEEN(6 5,90)))&CHAR(CHOOSE(RANDBETWEEN(1,2),RANDBETWEEN(4 8,57),RANDBETWEEN(65,90)))&CHAR(CHOOSE(RANDBETWEEN (1,2),RANDBETWEEN(48,57),RANDBETWEEN(65,90)))
Just press F9 to get a new value, or use a looping macro for your "... at least one character in the result would be a letter" restriction.
note: RANDBETWEEN requires the analysis toolpak.