System Restore fails

I am using Windows XP with a Dell Dimension computer. System Restore worked fine on my computer until recently. I could create Restore Points and Restore my system with it. I can still create Restore Points with it. However, the problem now occurs when I try to actually restore my system to a previous restore point. Now when I actually try to restore my system to a previous restore point with System Restore, I get a message screen (appears while the restore is booting), that has at the top of the screen, "Windows cannot restore to that restore point," and that I should, "try a different restore point." I tried restoring my system to three previous restore points but received the same message.
Not too long ago, I changed some of the drive letters for some of the drives. All of the attempted restores were to restore points created after I changed the drive letters, and had I rebooted since then many times. I don't know if the change of drive letters has anything to do with the problem of system restore now not working properly.
I tried looking in the Microsoft Knowledge Base but could not find a relevant article for this problem.
1) Why can't I successfully use the system restore feature to restore my system to an earlier restore point?
2) How can I get it to work again?
Thanks.