I need to update Windows through a limited account

My p/c has lots of problems and I suspect trojans for them, which cannot
locate besides, except for a Trojan Downloader.Java.Ope that found and erased
Netscape 8.
I have all kinds of hassle (clock changes time in an irregular way, access
denials, remote restarts, programs do not respond, windows serious error
recovering reports etc etc).
Microsoft Antispyware Beta also very often shows an alert saying my Internet
Zone level has changed below medium without me having done so.
I've found a way to update all my security programs (antivirus, firewall,
antispyware) through a limited account and if I update Windows too likewise,
I don't need to surf at all as admin, so this is GREAT (from security view,
I mean)!
I'd REALLY like to know how to update Windows through a limited account.
Also how to run SymNRT through a limited account.
I ran Trend Micro check antivirus a couple of days ago through a limited
account and luckily I had done so before but with administrator rights, so
the virus definitions were already downloaded.
Thus I ran the test successfully now through a limited account and it found
some malicious code in my system but "with no information about it" and it
said "if you delete it, maybe problems will occur" or so.
Didn't press the button "delete" on the spot and somebody removed the page
of the results.
Then I ran the test again and found nothing.
When I run any test through an administrator account either I'm not allowed
to finish it, (Symantec's check antivirus online for instance that runs only
rhrough IE, downloads for ever definitions and doesn't even start scanning)
or I'm found clear (like Kasperksy's check antivirus online that needs to be
ran for the first time with admin rights.)
But since I'm surfing through a limited account my clock NEVER CHANGES any
more!!!
(It's obvious this is due to a trojan, cause if I surf myself through a
limited account I'm not allowed to change the clock and in the same time,
nobody else is...)
So the question is how to do EVERYTHING NECESSARY through a limited account,
for now until to discover the trojan(s) and erase it(them) for good.