PI window encryption
Python - 370

Ok, nice one, finally got the pi thing working with thanks to link1 and link2.

from decimal import *
def f(s,n): 
 j=len(s)
 getcontext().prec=j+n+5
 d=Decimal
 e=d(0)
 for k in range(0,j+n+5): 
  e+=(d(16)**(-k)*(d(4)/(8*k+1)-d(2)/(8*k+4)-d(1)/(8*k+5)-d(1)/(8*k+6)))
 c=`e`.split("'")[1].replace('.','')
 t=''
 for i,l in enumerate(s):
  o=ord(l)
  for v in[0,32]:
   if 64+v&lt;o&lt;91+v:
    l=chr(((o-65-v)+int(c[i+n-1]))%26+65+v)
  t+=l   
 print t


Example output:

&gt;&gt;&gt; f('house',2)
isvxn


and another:

Wimt fcy d dnyh uhkvkv qhvadil   



       >>> f('This was a very secret message',1)