By Site Checker I assume you mean SiteAdvisor. That's optional so wont effect Security Center's behaviour.
VirusScan, however, is regarded as an essential component of any of the retail packages & has to be installed to avoid those alerts. Which means you would have to uninstall any other anti-virus that you may be using. Only certain applications are available as separate packages and VirusScan or Firewall Plus are not.
Out of interest what other anti-virus are you using instead of VirusScan?
I don't use virus scanners, rather I sandbox all applications that are potential virus vectors using a program called "Sandboxie".
Once malware infects your system you cannot trust anything your doing. If you delete the virus using a virus scanner your only putting a band-aid on a bullet wound. It's only delaying the inevitable. A compromised system can't be trusted. The only permanent fix is a full drive formatting.
I am a IT Security Analyst so I understand this is not for everyone. Average users should have a virus scanner and firewall. But for me, running anti-virus processes 24/7 is a waste of processors cycles and battery life. Behavior is 99% of your security.