1. As far as I am aware Avast will continue to support XP for a while
2 a) Exceedingly high as exploits for windows Vista and up will be fixed, but, this will highlight security gaps within XP
b) Yes, as a lot of XP's are still out there and becoming more insecure as time passes
On a side note I have seen very little "new" malware released over the last few months. I have a sneaky suspicion that it is being saved up for the expiry date
If you do not like 8 then at least upgrade to 7, skip Vista as it is rubbish