FOR, a good firewall and XP machines that cannot surf the net or receive uncertified software should be fine.
However, a few weeks ago I was reading a paper on how a bunch of smart hackers came to exploit a PDF file and make it launch executable code. Adobe has since patched up their PDF reader but you can bet that if something similar is discovered after XP support ends then Adobe *may* not provide an update for XP. This means that something as innocent as opening a PDF file may work ok on W7 but compromise your XP machines.