Cannot restore system to a previous date

I changed 3 systems during last 11 years due to strong trojan and invasion
problems in the past and recently I switched to broadband, hoping for some
better results due to the hardware firewall of the router.
I do not think I'm malware free though.
I have Windows XP, Norton Antivirus and Zone Alarm standalone firewall, also
some non-signature antispyware.
Recently, I had 3 incidents I believe show I have some problem.
At first my ZA suddenly started to ask if I want to grand access to the net
to the Symantec's ccSvcHst.exe, which I ignored.
Initially I thought it might be some malware and asked Symantec who replied
to uninstall and re-install antivirus, when I insisted if this might also be
some legal program, thay said it is the virus definitions update program. I
checked in the ZA program list and saw it is legal, so I simply ordered ZA to
remember allowing it always access to the net.
I believe someone changed the ZA settings hoping that I undertake the
uninstall reinstall process of NAV which is a big problem, if I recall well
from the past.
Then today my screen's settings changed strangely from 1024x768 to 640x480.
My admin account asked if I want Windows to fix that automatically but my
limited account I had to fix it through my screen's settings icon in the
taskbar.
Then I thought to restore system to a previous date to avoid eventual
malware but it had been imnpossible.
I tried several previous dates starting from the closest and proceeding to
the past, many times, through normal and safe mode, after seeing the
restoring procedure on the screen and entering again Windows I have an alert
saying "nothing changed in yr system, system restore could not be realized"
(I translate from local language as good as I can), "you may try another
date" and so on.
Additionnally I can think only of one thing relevant, I upgraded from NAV
2009 to NAV 2010 online, don't know if this has anything to do with all that.
Maybe if I restore system to a date before upgrading NAV, this could provoke
probems to the antivirus and thus it stops me from doing so?
Thanks for any help.