Do you mean that the batteries are 12V with a capacity of 7 Ampere hours (7 Ah) each?
If so then you can, as you say, tap in to a PCs' 12VDC internal power supply and charge the batteries in parallel.
Maybe you should be careful though at the very start of the charging cycle as a battery can take a few amps initially,
depending on how depleted it is to start with, it could be a good idea to only attach one battey at a time and let it
charge for a while before you attach the next battery.
Also, depending on the output of the power supply you won't get higher voltages than say 12 - 12.2 Volts, which
could prove to be too low a voltage as for example a fully charged car battery holds anything up to 13.5 Volts.
So if you start out with charging the battery to 12 Volts (or until it stops taking anymore charge) you could always
switch to using a joule thief to get it fully charged, the downside with that would be that it could take quite a while
to get the battery fully charged and you will also need to monitor the charge in that case, otherwise you could end
up frying the battery.
Here's how you turn a PC power supply into a lab power supply
http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-SupplyHere's how you build a Joule thief
http://open-source-energy.org/?tid=667Hope this helps.