http://www.bleepingcomputer.com/tutorials/tutorial56.htmlHowever you should pay attention to this part of it:
Problems with System Restore
There are some problems associated with System Restore when it comes to viruses. When restore points are created they are stored in a directory that is accessible only to the System account and not to a user. This keeps the restore points safe from misuse and tampering. Unfortunately this also means that any virus scan software you may have installed can not scan the files located there as well. This causes a problem if a file that is infected with a virus gets backed up into a restore point because now the anti-virus software can not clean it. Now if you ever restore from a restore point, that file that is infected will be introduced back into your system.
With this in mind, if you find that you are infected with a virus, hijacker, or spyware and want to make sure you do not get reinfected if you restore a restore point, you should turn System Restore off and then back on again to clear all the restore points. This will guarantee that their are no infected files that could be restored.
In short, System Restore is an easy and quick way to remedy SOME problems, but it has limitations. As Why Syzygy said, you should scan your computer thoroughly first and you are unlikely to have a restore point that far back.
Personally I think that if things are just too fucked up, the path of least resistance (assuming you have an XP disc) is to reformat and reload XP. Norton's is probably partly responsible for the slow down, this would be a good chance to get rid of it and switch to some of the free and highly recommended security programs.