Don't worry, it's okay. ;-)
.NET is a framework and runtime environment for programs which are based on it. There are differences between the main versions, therefore it's absolutely normal to have several versions installed at once, e.g. .NET 2.0, .NET 3.5 and .NET 4.5. It depends on which version is needed by a program. .Net 4.5.x is by default installed on Windows 8 and newer systems. Every peace of software on your system can cause security issues, but .Net 4.5 isn't worse than others. One of the biggest problems in this context is outdated software containing exploits, but as .NET is covered by Windows Update you usually don't have to worry about this.