Here are the steps you should follow to export to the cert on the existing server:

1) Go to Start -> Run and enter MMC
2) From the menu bar, select Console -> Add Remove Snap In
3) Click the "Add" button. Select the Certificates snap in, and then click "Add" again
4) Choose the "Computer Account". Click Next.
5) Select "Local Computer" Click Finish.
6) Now click "Close" and then "OK"
7) Now expand the "Certificates" object in the MMC and drill down to Personal -> Certificates.
8) You should see your existing cert. listed. Right click on the cert and go to All Tasks -> Export
9) Choose "Yes, export the private key". Click "Next".
10) At the next screen, leave the default settings and hit "Next"
11) Enter a password to secure the exported cert. Click Next.
12) Enter a file name. The cert will be exported to this file name. Click Next.
13) Click Finish.

On the destination server, follow this procedure:

1) Copy the exported file over to the new server
2) Follow steps 1-7 outlined above
3) Right Click on the certificiates folder and choose "All Tasks -> Import "
4) Click Next. Browse to the file you copied over in step 1. Click Next
5) Enter the password you entered in step 11 above. Check the box "Mark the private key as exportable". Click Next
6) You want to place the cert into the Personal store - this is selected by default. Click Next.
7) Click Finish.
8) Now you will need to go into IIS and assign the SSL cert to the site.

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