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You just need to transfer a license file by using "Reconfigure Software Licenses" option on PTC's site, here: http://www.ptc.com/appserver/lm/rsl/reconfig.jsp.
Also, when creating a request for a new license, you must specify the new MAC-address (from a new PC).
You can install Mathcad and activate license online using your product code.
I hope above steps will help you.
Thanks.