Hi.
If the current OS install is from a time before the system had SSE capable processor then that might be a reason.
Usually it is recommended to update BIOS to latest version, then a fresh install of OS to have the SSE picked up.
There are a couple of CPU stress testing tools that will detect if SSE is enabled.
e.g Hot CPU Tester Pro, StressCPU
http://www.ultimatebootcd.com/
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