…but it won’t do everything.

For example, and this is something good, it won’t erase your emails and personal documents. But you should still back up all of your information. Another thing it won’t always do, and this might be something bad, is remove all of the programs installed during that time. You might have to use the ‘Remove Programs’ function from the control panel.
If a system restore doesn’t help your problem, and the virus removing software doesn’t work, then you might be faced with reformatting your harddrive. But system restore seems to take care of about 90 % of computer problems…so it’s better to try that before going through the headache of reinstalling everything.