That all depends. A virus can usually only be detected by an AV program once it has been updated with definitions that include the signature of the virus in question. Until that time, it is pure speculation as to when you acquired it. Although AV vendors are very quick at releasing definition updates, there is always a delay between a virus being found in the wild and the definitions being updated (and subsequently downloaded to your computer). This is the danger window, and it can be a matter of minutes, hours, or even days.
Using a System Restore point will, as Richard points out, make you virus-free. However, I would be inclined to use the point before the one that was created before the virus was discovered. For example, if the virus was discovered on Day 3 and you had restore points on days 1 & 2, I would use the older point (i.e. the point from day 1). In any case, I would perform a full scan with my AV software after restoring. Don't forget to update the definitions first, as these will also be rolled back by the restore.
