Well process explorer shouldn't really be running all the time in the background. You should only run it for a specific purpose, e.g. fault finding, looking for unknown process or something using a lot of CPU (but that can to all intents be found using the Task Manager for a lost less system resources.
When you are talking about only 512MB of RAM you have to be economical with running processes or you are going to be constantly swapping elements loaded into memory to the pagefile, this activity can cause avast to scan increasing cpu use, not to mention all this swapping in and out of the pagefile will have your hard disk working overtime.
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That is me for the night, 4:25am here.