In Windows 7 and Windows Vista you shouldn’t need to uninstall previous versions of .NET framework such as .NET 3.5 and .NET 4.0 because those dependent apps on that Framework may fail to work.
.NET 4.5 System Requirements
- Windows Vista SP2 (x86 and x64)
- Windows 7 SP1 (x86 and x64)
- Windows 8 (x86 and x64)
- Windows Server 2008 R2 SP1 (x64)
- Windows Server 2008 SP2 (x64)
- Windows Server 2012 (x86 and x64)
Hardware requirements
- 1 GHz or faster processor
- 512 MB of RAM
- 850 MB of available hard disk space (x86)
- 2 GB of available hard disk space (x64)
- 350 MB of available hard disk space (language pack)