Unless you created a restore point, you cannot go back to it.
If you have an image of your original install, then you can install image.
You can make a new install, however, you will need your XP install CD or copy on HD.
F8 is not for doing a factory restore, it's only startup options if Windows has issues.
Based on your system, you will have either a recovery partition you can enter with a key when the system is starting (F2 or something), or a set of CD/DVD disk to restore from. This will erase all of the data on the disk though, so don't do this unless you have a backup of your files or you get a new drive and restore the OS on that (which is what I recommend).