steam's Steam Guard

I just recently reinstalled fallout new vegas on my pc (because i had to reinstall my copy of windows vista home premium 32 bit) and Steam Guard asked for permission to allow it to lock my account to my pc, im just asking if anyone knows if thats safe, and what if i upgrade my hardware or my os??:cry
Because i do plan on buying windows 7 when i can afford to

 

Should be fine. I have steam installed on 2 PCs. If memory serves me correctly they email you an activation code to make sure that it is you trying to access steam not someone trying to hijack your account.