CanNot Enable On Access Scanner

i installed your Comodo Anti-spam product and haven’t used it yet, so I want to Thank You for such a Good Worthwhile product. I heard about it a long time ago and have been wanting to try something like that. Nows my chance!
Now for my Problem… As I have said in the Subject - I canNot enble the Enable On Access Scanner. I see the red Sheild down in my System Tray and I right/click on it and it just Won’t Enable it. The little tiny message keeps saying - To protect your system against viruses enable On-Access Scanner. I WISH, but it just won’t work ---- I’ve right clicked on it about 4-5 times and a Error pop-up window pops up and says Error - Comodo Anti-virus encountered an error and then under that it says this:
Number - CAV003
Description - On Access Scanner failed to start…
and there you have it. So how do I Enable it so it will Work(scanning my computer from viruses/trojans, etc.?) I have XP Media Center plus Process Guard working - I uninstalled any Anti-vius software before installing and temporarily disabled Process Guard while installing and untill I could Enable your Software. Still didn’t work! Everything on your Comodo Anti-Virus works Good but Not the Anti-virus itself? Thank you in advance for any help.