For an experienced computer user, System Restore is just a convenience. If your computer doesn't like an update or you bork a system file, System Restore is a quick fix. Used incorrectly, like the monkey said, it can reintroduce a virus. SR is often accused of "not working" because it will indicate that "no changes have been made to your computer", but SR only monitors certain key system files and extensions. If there is nothing to "restore", it won't, and it does absolutely nothing to data files. This is good because you can restore to a week ago and not lose your current work, but it will not help you get back your email or deleted item.
Frankly, it doesn't hurt to leave it running, just in case, but, as always, prevention is a better "cure" for malware.
(we want more baby pics!)