I just downloaded and installed AVAST on my VISTA machine, but would like to put it on my XP desktop as well as my eeePC. Can I use the same registration code for all three machines?
Yes … as long as it is the Home version and no computer is used commercially.
I don’t see my response which was a simple Thank You,
The answer is yes IF you’re using the three computers for home and non-commercial.
ALWIL won’t like it but you could use also another registration code as it is free and easier to receive on each computer if it’s not on a Local Network (my case). It’s FREE, after all… 
I managed to get it registered with the same code, but I couldn’t keep it on my eeePC because the PC has such a small drive only 4 gbs. that I try to put as much as I can on the removable SD card and was able to run AVG with it installed on the card. I am going to post this as another question though.
Can’t avast be installed in the removable card?