Vista - System Restore Problem?

I have a new PC with Vista Ultimate. I recently had some error messages when
I switched on my PC (Display Driver R300 stopped responding, and has
recovered) - firstly, is this a serious problem?
Secondly, I tried to do a System Restore to the point before these problems
began, which was 5 days ago. There was a System Restore point for each day
when I switched on my PC since that time. However, I could not restore to
the point 5 days ago, despite the fact that I have not made any changes,
installed any software or updates, and only used my PC for email and
Internet. Why can't I go back to those previous settings? Is there a
hardware or software problem? Thank you.
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Thanks very much for your help.
Steve