AVG is notorious for this and has been for years. Microsoft has their built-in Windows Defender which I use until something else better comes along. As a paranoid former IT guy, I run Malwarebytes and Super Spyware Hunter manually once a week to check for anything else.
Private memory is that which is used by programs, and will go up and down as a program uses the memory. You shouldn't be too concerned about this unless your CPU load shoots up and remains at 100% at the same time as that would indicate a possible malware infection.
You might want to run Process Explorer (run as Admin too) because you can break out the memory usage better and even suspend applications that you think are suspicious and kill them. This is unlike the Task Manager which gives mostly useless cumulative numbers, which are useful for a distance glance at an underlying problem if there is one.
For memory usage overall, I suggest this nice seminar demonstration by Mark Russinovitch.
https://channel9.msdn.com/Events/TechEd/NorthAmerica/2011/WCL405
This is well worth the time spent watching this. Also watch his other series on malware because there's some interesting tips in there too on using Process Explorer and Process Monitor. Sadly, as confirmed by Malc, RAM Map is broken until an update comes out.
John