JMH said:
Re AVG slowing down a system...
Don't use AVG on dinosaurs.

We find AVG Pro fast, efficient & non intrusive. [ including LinkScanner]
I do have a dinosaur and I was using AVG free, not Pro. I can only attest that AVG slowed my system down scanning downloaded files (like BSOD dump files). It would take three to five minutes to scan these files. LinkScanner worked well. It was slow at scanning incoming email also. Other than the slowdowns, I had no problems with AVG. I am now running AVAST and there are no slowdowns at all.