select
	Random_String =
	substring(x,(abs(checksum(newid()))%36)+1,1)+
	substring(x,(abs(checksum(newid()))%36)+1,1)+
	substring(x,(abs(checksum(newid()))%36)+1,1)+
	substring(x,(abs(checksum(newid()))%36)+1,1)+
	/* and so on for as many characters as needed */
	substring(x,(abs(checksum(newid()))%36)+1,1)

from
          (select x='0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ') a
     
Results:     
Random_String
-------------
T7TAR

with
a1 as (select 1 as N union all
       select 1 union all
       select 1 union all
       select 1 union all
       select 1 union all
       select 1 union all
       select 1 union all
       select 1 union all
       select 1 union all
       select 1),
a2 as (select
            1 as N
       from
            a1 as a
            cross join a1 as b),
a3 as (select
            1 as N
       from
            a2 as a
            cross join a2 as b),
a4 as (select
            1 as N
       from
            a3 as a
            cross join a2 as b),
Tally as (select
            row_number() over (order by N) as N
          from
            a4)
, cteRandomString (
    RandomString
) as (
select top (64)
    substring(x,(abs(checksum(newid()))%36)+1,1)
from
    Tally cross join (select x='0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ') a
)
select
replace((select
    ',' + RandomString
from
    cteRandomString
for xml path ('')),',','');