One Win7 copy for 2 Pc´s?

Hi,

I recently built myself a new pc. To save some money I thought I might as well use the Product Key from my old Pc´s OS (Windows 7 Home Premium 64-bit), which I did and it works normally BUT: Later on I wanted to use my old Pc again but I hesitated before turning it on as I was not sure wether MS would sue me or something because of using 1 copy of Win7 across 2 Pc´s :P but I would only use one Pc at a time, Not Simultaneously!

So my question is:

Can I use the same copy of Win 7 for 2 Pc´s as long as I am not using them at the same time?

Thank you in advance!