Those two that are not updated, are they connected to the internet? Are they connected to a home network with other internet capable computers? Do users insert CD's or connect external media where infected files may be present?
If the answer to 'all of those' is No, then no need to update. if they are completely isolated from possible infection sources, then there is no need to update them.
If the answer to any is yes, allow Windows to be updated on each.
I work with 8 identical PCs at work and at home, they all have same Win XP Pro (bulk license), 6 update automatically, 2 have never been updated.
This 2 un-updated, still work fast, (as if windows was freshlly installed) the other 6, keep getting slower, & sluggish. In 4 of them Windows was reinstalled 2 and 3 times; and the remaining 2 became ultra-slow.
Software and use is the same on all of them.(office work) No additional software downloaded or installed in any.
Should I forget about the "risks" of not updating and reinstall and use the no update option???