There's a little confusion on what the terms mean.
First off, when Windows shows a connection at 144Mbps, it refers to 144 Mega *bits* per second. Notice the small "b": Mbps.
Data transfers however tend to be measured in Mega Bytes per second: MBps - See the big "B"?
Wireless speeds for 802.11g top out at a 54Mbps data rate. But actual throughput is about half that, 25Mbps at best. Convert that to MBps and you get something around 3MBps. This is on a good day, with the wind at you back on every other 2nd Tuesday in leap year.
If you share the wireless connection with other devices (Like another laptop), all bets are off. It can and will be considerably less. Also take into account interference from other wireless routers, other devices like cordless phones in the same band, microwave ovens, etc. and the number goes down even more.
In your case, the bottleneck is Computer B with it's 802.11g connection. Since you already have an 802.11n wireless router and 802.11n on the laptop it's worth it to swap the wireless card on the desktop or just wire it directly to the router if you can. Be advised though, in order to get full speeds on 80.211n the ethernet connection to the router must be Gigabit Ethernet.