I’ve seen lots of ads for pcmatic.com as well as a couple others that must be very similar. Are they just lite versions of traditional anti-virus software?
Not exactly
Many of them will also scan for things like missing or damaged copies of windows files and can replace them. I have a subscription to a similar service that:
Tests hard drives and ram, checks for viruses, looks at a few tens of thousands of thousand registry entries for sub optimal settings, and checks all windows system files for if they match a current copy of windows as of the last service pack. If they do not match, it replaces it with a fresh copy. Does XP Vista, and 7 32 and 64 bit versions. If you are and advanced user with specialized needs they can mess with your settings, but for joe schmo user a pass with such a service every 6 months or so is actually very useful.