Posted 05 September 2009 - 10:21 AM
Some viruses can be very difficult to analyze. A fairly common feature of sophisticated viral attack is that they play games with your security... Simple things that you can easily check and which are a good indication of infection are:-
Check and see if both the Firewall and the security centre are turned on. Do this in a sneaky way by going to "Control Panel" >> "Administrative Tools" >> Services and scrolling through the list. Both the Security Centre and your Firewall should be marked as automatic start. if they are stopped, start them, (while making them automatic,) and see what happens. Check and see if you can revert to a previous system restore point and create new system restore points. Both of these can be accessed through the Windows Help and Support Centre. (I am much more familiar with Xp than Vista, so you will need to use windows help and support for the exact details anyway.)
The important thing is that in order to evade security, malware has to attack these key systems. There are registry keys you can look at, but if the malware has any kind of sophistication it will create a virtual registry that appears to be clean. So without using specialized tools, us mere humans have to be sneaky and look for inconsistencies.
Your anti-viral scanner, (I use Avast,) should check the MBR, (a common hiding place for these viruses,) check the scan logs to see if the MBR was actually scanned?
If these checks come up positive, then you probably have viral problems, if they don't, you still might have viral problems but personally I would look for other solutions first, for example:- "am I remembering to click 'Apply' after changing the settings?"