Here is MS Policy, if you have purchased OEM / Retail from a local software vendor, and have lost the Installation Media you may get in touch with Microsoft Connect...
One is expected to be registered to MS with the copy of his Windows...
One can call MS, at these Numbers for Replacement media...
1800 102 1100 (Toll Free number – Accessible from anywhere in India from Airtel land line and mobile phone)
1800 111100 (Toll Free number – Accessible from 36 cities in India from MTNL/BSNL landline)
+91 80 40103000 (Toll number – Accessible from anywhere in the world from any telecom)
the replacement takes MAX of 90 days.... usually lot quicker...
But if the Windows copy came pre-installed then one needs to contact his system builder (like HP/IBM/Dell)....
P.S. The replacement disk may not be free
one may be charged for the same...