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We’re getting off-topic here, but just for the record – 486 started (back in 1989) with the DX version on 25MHz (together with stronger 33 and 50 MHz variants), soon to be followed by the weaker 486SX model (which was running as low as 16 MHz in its basic configuration).

Of course, later on, 486 frequencies went much higher (up to 133MHz if I remember correctly).

But anyway, if the original poster has a 400MHz processor it should be way enough horsepower to run avast (but keep in mind that CPU frequency is not everything - size of RAM tends to be a key factor in these older systems).

Cheers
Vlk

P.S. for a brief history of the Intel CPU’s see here: http://www.intel.com/pressroom/kits/quickrefyr.htm