I've found that on old PCs with Pentium III processors (generally dating from 2001 or earlier) Avast is better than MSE. The problem with MSE for such PCs is that the anti-virus updating really does grind such a PC down for several minutes. I agree about using the Windows built-in firewall.
Avast for me also if you sail close to the wind. On my partner's netbook MSE is good enough as she only goes online to shop. (Known sites) Avast is more protective when doing searches through engines. This AV backed up with SAS.
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