Sorry I can only answer the first question, avast works fine with 64 bit versions of Windows.
The others should really be on the General forum as they are unrelated to avast. I would say however, that rather than install something and have to go back, which is likely to be more of a nightmare than a dream. You should investigate fully what it is you want a 64bit OS for as there many issues you may not like, such as finding software that will work on it, getting drivers for your peripherals, etc. etc.
The other aspect is yes it may be possible to go back with vista business, but does vista basic meet your needs. Many of the functions a home user would like are not in the business version, like media stuff. You really need to do your home work and choose the OS that best suits your needs.
