Sorry there is a misunderstanding here. No matter which version of Kaspersky AV or Kaspersky Internet Security you buy the serial number will work with the latest version regardless for the entire duration of the license. I've been using KIS for around 3 years and this has always been true.
I strongly recommend that you investigate Kaspersky Internet Security over just Kaspersky Antivirus. It is excellent and by far the best combined suite I've ever used. I bought a 3 user license for a year for £13 last year so it costs little more than just getting the antivirus.
Cameraman:
Just be aware that it might not be the latest version! I have Kaspersky Internet Security 2009 ,5 licence) for less than £40 last year, and it is very good indeed. in fact I am about to buy another year's subscription within the next week. I will let you know more when I have it if you like