If all you are doing is reinstalling on a HDD for the same compute, Most likely - No problem. Activation takes into account several factor such as HDD, MB(ie chipset/BIOS), GPU and 10/100 chipset. Changing just one item normally does not trigger a call to uSoft.
I have done several re-installs of Win 7, both desktop and laptop, and activation went thru without having to call. For my laptop I have a HDD and a SSD both with the same Win 7 Activation code. The 2nd installation on the new SSD did not require a call. Have too swap Hard drives due to a conflict between O'scope (Serial port interface) and to download temperature/Humidity data (also requires serial port). The two software programs conflict.
The only time I had to call was on a install of uSoft office (3 user Limit) - went smooth. This was on my wifes vista machine. I later replaced it with a new win 7 system, when I installed office, I again got a message to call. I decided to wait, a coulpe of days later I went back and tried to activate ( to get the message and phone nr) to my surprise - it activated. I think there is also a time interval included