It depends on how you got it. If it came per-installed on a system (OEM) it's not re-usable, if you bought a Retail version, AND disable/deactivate the 1st install, you can use it on another system.(may take a call to microsoft to activate)
I built my pc in November last year, self built a have the OEM windows 7 home premium and was wondering if it reusable?
OK. You have an OEM version. That is tied to the first motherboard it is installed on. You might get Microsoft to authorize use on different hardware, but they are under no obligation to do so.
And as always, one license, one PC.