Have you tried concatenating the now function and a random number ?
= text(NOW();"yyyymmddhhss")&RANDBETWEEN(1,1000)
Now() is the time now, but excel gives that as a number of days since the
1st january 1900 or something similar.
The text function will allow to format that number as a text string with the
year, month, day, hour, minutes and seconds.
If that is not sufficiently unique you can add on a Randbetween function
which will give you a (pseudo)random number.
Randbetween requires that you give the upper and lower limit for the random
number.
If you can't see the randbetween function in excel then you will have to add
on the analysis .xla.
I can't quite recall waht it's called in English.
But someone else can help you on that if necessary.
Seamus