MSE does take a lot of RAM, and sometimes takes a lot of CPU time. Sometimes it slows down my laptop, which also has a dual-core CPU and 4GB of RAM. But it reportedly takes less out of your computer than many other anti-virus programs, and it's programmed with Microsoft's current best practices. That makes it less vulnerable than many other anti-virus programs. Also, it's free, and integrates with Windows's built-in automatic updates.
I think that keeping the old computer for the monitor is a poor decision, unless you have no other monitor for it. My laptop has an adequate GPU, and I attach a 20" monitor to it. I'm sure your laptop can run its 17" screen and your 22" screen simultaneously, and the edition of Windows 7 will permit an extended desktop, so you can get a really wide view.
Or, my monitor is an IPS display, so I can rotate it and get a really tall display for reading and writing. I keep my email and status indicators open on my laptop's screen, and type on my big screen.