Note NetProtectPlus is a full internet security suite, whereas most "free" anti virus packages are just that, for viruses only.
Computer security is FAR more than just viruses, there is spyware, malware, phishing and so on.
A free A/V product wont protect against them.
p.s. Probably the most dangerous thing you are running is Windows XP as Microsoft are now no longer supporting it or updating it with security patches so it will be wide open for security issues.