Let's look at Norton. Just one of many programs that interfere with SR.
-> READ the section titled "Many users have found that Norton 2006 and later, has been responsible for this error message. It seems there is a security feature in place that prevents outside programs from making system changes that would change Norton's files. To work around this issue, follow these instructions:" at http://www.5starsupport.com/faq/xp-system-restore.htm#1-21
This is just one of some hundred reasons it doesn't work. Often I need to see the machine first hand to find out why but this one comes around too often. Makes you wonder what Symantec was thinking.
Anybody who can tell me why the restore in windows xp does not function most of the times and sometimes never ?