Sallyanne,
my setup is similar to your.
I am running GIMP 2.8.14 here on a 4year old laptop with Win7 64bit and 4GB RAM without any problems.
I also use avast, so I don't think that this is the problem.
In the beginning, my Gimp used 200-300MB of RAM. Now it uses between 500-1GB. This is most probably due to a lot of heavy brushes I have installed, so it is a good option to move them out of your brushes directory and "move them in" when you need it. I also noticed that Gimp uses more and more memory when being used over a long time (e.g. on complete weekend day with heavy tutorial etc. working

) and playing a lot with filters, undo/redo and other features. I don't know whether this could indicate a possible memory leak or not, and I have not tried GURM yet.
If you are running on Windows, I would try the following things:
* backup your Gimp home directory (.gimp...)
* do a clean uninstall of gimp (e.g. with REVO uninstaller, which also tries to sweep out left files not catched by the uninstallation routine)
* delete your Gimp home directory (remember the backup!)
* Use CCleaner to clean your hard disk from temp files and other "windows-paralyzing" stuff, occasionally repairing registry errors.
* See if you have enough left disk space on your windows drive.
* Check in control center -> system, whether your swap space is automatically handled by windows (recommended). The used swap space can be more than your RAM size, if needed!
* reinstall Gimp
* Start Gimp without any plugins, scripts, brushes or other addons and see whether it works and how much RAM is consumed.
* Install your addons one after another. After each, restart Gimp and check RAM consumption and whether Gimp still works, so that you have a chance to catch the erroneus plugin or whatever might lead to the system crash.
There are a lot of further tests, system enhancements etc., but I hope this will work.
dietmar