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Details for a Dell Inspiron 1545 factory recovery here: How To Restore or Reinstall Microsoft Windows on a Dell Computer | Dell
Providing you haven't deleted the recovery partition on your hard drive you should use Step 2: PC Restore.
If you have deleted the recovery partition on your hard drive you should use Step 3: Manual Reinstallation.
If you haven't made a set of recovery discs you can order a set from Dell, failing that, if you know someone whose got a Windows 7 disc you can use that to reinstall Windows using the Product Key attached to your computer.
Clean Install : Factory COA Activation Key