System Restore Criteria Question?

In the past, I have noticed in XPPro that some system restore points can be restored, and others cannot. It goes through the hold routine saying that it is restoring files, shuts down, boots up, but when it boots up for the final time, the message is that the restore point was not restored and nothing has been changed on my computer. Would I like to try another restore point?At the moment, none work, including three from today.Does anybody know what the limiting factors are to this provision, or how to get around them?Thanks:RTH