I think my Dell laptop is infected with a virus/spyware but it can not be detected?

I have a Dell Inspiron laptop with Windows XP Professional SP3 and starting few days ago I accidentally noticed the following:

1. the Automatic Updates feature is not operational. When going into the Windows Security Center, I noticed that the Automatic Updates feature is turned off. When I clicked on the “Turn on Automatic Updates” tab I get the following message : “We’re sorry.The Security Center could not change your Automatic Updates settings. To try changing these settings yourself , go to System in Control Panel. On the Automatic Updates tab, select Automatic (recommended), and then click OK “. Then I went to the System - Automatic Updates : I noticed that the “Turn off Automatic Updates” tab is checked/ticked and that this tab and all the other tabs [ “Automatic (recommended) ” , “Download updates for me , but let me choose when to install them” and ” Notify me but don’t automatically download or install them” ] are not active - that is the headings of the respective tabs are translucent, and therefore I can not check/tick any of the three tabs for turning on the automatic updates.
2. I tried to do a system restore using several earlier dates but it failed.
3. The letters “i” and “l” of the name of files within the folders appear with BROWN color.

I did ful scans using Avast, Spyware Doctor, Spybot Search and Destroy and Norton Security Scan but no virus or spyware were detected.

I should add that a couple of weeks ago I started to use Windows Steady State but 3-4 days ago I decided to remove it because it takes to much time to do the changes when restarting the computer.
I also used Systernal Rootkit revealer but nothing significant was detected.
I would appreciate a solution to my problem.
Thanks !
I should add that so far I managed to successfully download and install mannually all the updates using the Microsoft Update web site.

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