It is one of the problems with 64bit OSes there are much less in the way of programs that support it, but it is getting better slowly. Fortunately avast works on it but that one function which works on the 32bit versions of windows doesn't work on the 64bit version.
Even though XP 64bit has been on the scene for some time the software companies just don't see the value in developing 64bit versions or getting it to work with the 64bit OS, as I guess the number of customers is low.
That problem is however, not the fault of MS the 64bit OSes have been out for some time. There will be many people on this forum will find it strange me defending M$. Part of that problem is that when people buy new systems and an OS they may not receive the best advice on what flavour of windows XP/Vista, 32bit or 64bit version from the retailer.