James:

Restore points are maintained for up to 90 days (but you wouldn't want a
90-day old restore point unless you haven't used your computer for 89 days.)

There's no need to use up to 18GB of your hard disk for restore points.
That's overkill and, quite frankly, silly.

Let's see if I understand you: You created two restore points, restored
to the earlier one, and now you want to know why you can't restore to
the later one?
---
Leonard Grey
Errare humanum est

Hi James,

I would first reduce the amount of disk space System Restore is using to
hold restore points to 1GB.

Then try running System Restore in Safe Mode.

Boot into Safe Mode and perform a test restore using the following link.
http://bertk.mvps.org/html/tips.html#3

How to starting Windows XP in Safe Mode:
http://bertk.mvps.org/html/safemode.html

If there are any Norton applications installed, take a look at this page.
http://bertk.mvps.org/html/symantecdoc1.html

Here are some more troubleshooting steps to take when System Restore fails
to restore:
http://bertk.mvps.org/html/srfail.html

The Event Viewer can sometimes be helpful in tracking down errors.
Using the Event Viewer to troubleshoot System Restore errors:
http://bertk.mvps.org/html/source.html

Regards,
Bert Kinney MS-MVP Shell/User
http://bertk.mvps.org
Member: http://dts-l.org

Three points:

1. System Restore fills up the space allocated for Restore Points.
After that happens the next time a Restore Point is created, the
oldest one(s) is deleted to make room for the next.

2. There is a maximum of 90 days longevity for any Restore Point.

3. 90 days is *way* too long for a Restore Point. In practice you can
seldom go back more than a week or two, because any more than that and
your registry gets out of synch with everything else. So allocating
space for more than a dozen or so restore points is a waste.

Read here: http://bertk.mvps.org/

That's *way* more than is needed. See point 3, above. 1-2GB is plenty.