Here is another way.

Put this in A1 (or give it a Name):
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz

Then copy this to A2:
=MID($A$1,1+INT(RAND()*LEN($A$1)),1)&MID($A$1,1+INT(RAND()*LEN($A$1)),1)&MID($A$1,1+INT(RAND()*LEN($A$1)),1)&MID($A$1,1+INT(RAND()*LEN($A$1)),1)&MID($A$1,1+INT(RAND()*LEN($A$1)),1)&MID($A$1,1+INT(RAND()*LEN($A$1)),1)&MID($A$1,1+INT(RAND()*LEN($A$1)),1)&MID($A$1,1+INT(RAND()*LEN($A$1)),1)

Ola Sandström