This behaviour is not limited to avast, virtually every AV company when giving indtruction in removal of a virus, warn that in XP you will probably have to disable System Restore because it is protected (windows system, hidden, etc., tec.) and going to be back.
If you are having problems getting the scan at boot I suggest that you download RajZors
avast! External Control Tool and run/set it up from their.
I hate System Restore - I see so many people get into a complete mess, going from one restore point to another, to last known good configuration, etc. that they don't really know where there system is at. Resulting in not knowing if the latest windows.updates are applied.
I too would recommend a third party backup/restore program. I use Drive Image 2002 and it takes a full image of my C:, D: partitions and saves it to my second HDD. If I have a problem I copy back the last image. Another Imaging tool is Acronis - True Image 7.0, this has much more functionality than Drive Image 2002, such incrimental image backup... This cuts down on the image creation time time.
The link to show how to view and access System Volume Information, I believe could be more dangerous than disabling SR. MS doesn't got to the kind of trouble to hide it for nothing. This should carry a health warning.
HTH David