Security Scan Plus is an optional download with various products, most notably Adobe Flash, Shockwave, Reader etc. (example below - click image to enlarge) and if you aren't watching what you are doing and fail to uncheck that option, it installs.
Many other software types can be accidentally installed this way if you don't take notice. It's a very popular method of keeping software free or low-cost by carrying advertising for other products.
It's harmless and merely checks your machine for security flaws. You can uninstall it via Control Panel > Programs > Uninstall a Program (Add or Remove Programs in XP).
You don't need either of them as you should have proper antivirus and firewall software installed. Uninstalling as in the previous line should clean out both instances. In any case I think those 2 instances you mention are for the same software.