I have got such this issue with different antivirus softwares.
for example, I install an antivirus software (assume this computer has just internet access),
then I make full computer scan, (so the computer will be clean),
after couple of days or one week, when I again make full scan, I observe some new viruses and spywares.
100% sure that these viruses came through the network adapter.
so the question is, why the antivirus do not detect these infections until I make manual scan !??
Automatic scans sometimes don't catch everything, because the settings for that are designed to require minimal CPU usage so that you can continue using your PC while the scan goes. Deep scans require much more resources. On a separate note, a few days is plenty of time for new things to pop up for a variety of reasons. You may have downloaded an infected file, or visited a dangerous site and ended up with some malware. Your virus databases are frequently updated as well, in order to catch new things that it might not have been able to detect on the last run.
There can be several reasons for this. First off let me say that no antivirus program catches EVERYTHING. For example an anti virus program often focuses on viruses and not malware like adware. This is why i often suggest running an anti-virus program AND a malware program like Spybot: Search and Destroy.
Second, viruses often bury themselves deep. They start running when windows is running and if your anti-virus picks them up, it may be that the are only picking up what is running. The rest of the virus could be still hidden and inactive until the virus realizes it is not active then spreads again. The best way to run anti-virus and malware scans is from Windows SAFE MODE. This way most viruses are not activated and cannot protect themselves from being discovered and are more easily removed.
So that is the first thing i would do, is download Spybot:search and destroy (google it as it is free). Boot into safe mode and run both the anti-virus and spybot. Reboot into normal windows and see how it goes. If they come back, at this point i would suggest backing up all your items you need saved, format the drive and reinstall Windows as this is the only 100% way to get rid of viruses.