Can Avast remove virus “Boot.Mebroot” from already infected PC with Windows XP? If it can, I will buy it.
That is an MBR infection, no?
I do not know if any resident AV can clean those, in my experience they require special tools or replacement of the boot record/OS reinstall. I could be wrong though.
On a side note, even if Avast! could clean it, there would be no need to buy it, as the ability to clean an infection is no different from the Free version to the paid ones.
Now, if you have a PC infected with that, we have guys here who will clean it for you, gratis, head over here to see about that>>http://forum.avast.com/index.php?topic=53253.0 , then post the resulting logs back here when the tools are finished.
I printed out all of the directives on the page link you supplied but if you actually have or know someone who could help me fix this for free, as you say, I would be indebted forever. This “Boot.mebroot” virus has me petrified !!! I’'ve disconnected the PC but afraid to reconnect. Willing to try anything that may solve this
Yeah, they will scrub it for free, they enjoy killing malware. You don’t need to thank me though, save that for the experts when they are finished 
Mind you, it can take a little time, because of time zone differences, and they take multiple steps, meaning you will be told to re-run the tools with specific commands, sometimes multiple times (just browse this section of the forum for numerous examples).
But yeah, give those tools in that thread you printed a run, then when you post the logs back here, once they read them, they will tell you what to do next.