I just checked - at the moment I have 5 PCs on bitdefender, 3 on a "total security 2011" which i bought around xmas, and 2 on an "internet security 2009" which I bought in 2009 but hadnt used (i'd grabbed a couple boxes in a deal somewhere and one was meant for my parents but never used). The PCs on the 2009 license code are happily running IS2011, and I am pretty sure that if I install the 2012 it will still happily run. So it can be bought in advance and used later, typically. Note that a 3 PC version will tick down as soon as you put the first PC on it, though
it is unclear to me what the difference remains between "internet security" and "total security" in the 2012 edition - file delete, encryption, tune up (there is a check-optimize which is in both TS and IS 2011 and not as dumb as it sounds, but not sure what tune-up is meant to be) and 2Gb back up (pff). The difference in price is not much for 3PCs but it also seems to be extras that one already has elsewhere, most of us (Note that BD also has a "master control" option where a central PC can oversee other PCs in the home/home office network. Some of these functions might be more valuable in such a scenario)
PS: the PCs I have on it are a mix of XP 32 bit, windows 7 32 bit and windows 7 64bit.
PPS: the program is not as fast as, say, Eset used to be (havent tried it recently), it's a little on the slow side but doesnt cause too much load in that. Game mode and laptop mode are ok. It does work and it is cheap compared to nearly any other product with the same kind of security track record.
Note: The program is VERY disk space hungry, easily 1Gb or more. I think it keeps a file signature database for heuristic detection, and it keeps many versions of it... that's probably configurable somewhere