Hi Gene54 and welcome to the forum.

We have seen a number of people on the forum with conflicts with ZA and Avast. That is why it was suggested above that you uninstall ZA (not disable it) from your machine, then boot. This will free up much memory.
You also don't need both MBAM (Malwarebytes) and SAS (SuperAntispyware). Everyone has their preference, but most prefer MBAM since SAS clears many cookies and CCleaner (see below will do that for you anyway). So by deleting one of these, you will also free memory. Also, don't save any .exe files, documents, pictures, or music on your machine; put them on a flash drive to save memory instead.
After doing the above, use a cleaner to clean out your system with
CCleaner Slim version:
http://www.piriform.com/ccleaner/builds - scroll down to 4th download option. It removes unused files (cache, temporary Internet files, etc.) from your system allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. Additionally it contains a fully featured registry cleaner (if unsure of cleaning registry, make a backup folder in My Docs temporarily prior to fixing).
In addition, since you did an upgrade from 4.8 to 5.0.594 and you are having problems, it is suggested that you uninstall and do a CLEAN (fresh) install of Avast:
1. Save a copy of newest version of Avast (5.0.594) for the version you need and save it to your HDD:
Free –
http://files.avast.com/iavs5x/setup_av_free.exe 2. Download the Avast Uninstall Utility, aswClear5.exe
http://www.avast.com/uninstall-utility and save it to your HDD (it has uninstall tools for both 4.0 and 5.0).
You will need to uninstall BOTH from your machine.3. Disconnect from the Internet at this time.
4. Uninstall Avast through "Add/Remove Programs" through Control Panel if possible.
5. Boot into
Safe Mode (hit F8 repeatedly) and run the Avast Uninstall Tool.
6.
Reboot twice - esp. for XP machines.
7. Install the newest version of Avast and
reboot.
8. Get Internet access and update Avast definitions.
9. Register your copy for Free –
http://www.avast.com/registration-free-antivirus.php. Reboot.
After this, run CCleaner again, then defrag your machine. If you don't have a good defrag and memory is now improved,
free Puran Defrag offers a defrag as well as a boot-time defrag, which works well:
http://www.puransoftware.com/Puran-Defrag.html.
Let us know how this works for you. I've tried to give you suggestions for your problem but also to help you with your limited memory resources.