ScanDisk is a builtin tool in Windows which detect and fix any potential damages in your hard disk. It can improve the hard disk reliability and performance.
Computer Virus may runs different processes which can slow down your machines. Removing virus not only protect your machine but also improve the overall performance of your machine.
Spyware/Adware are similar to virus. They may run different processes to monitor your machine or display advertisement. Removing Spyware can definitely improve the speed of your machine.
The scattered data in your hard disk will increase the read/write access time. Defragmentation will rearrange your data in sequential order which can improve the overall speed. Defragmentation should also be run against the PageFile in Windows. There are some freeware which allows you to defragment PageFile.
Run the software on startup is useful. But it also takes up machine’s resources. Check if any software is no need to run on startup and stop it.
Uninstall unused software can speed up machine’s performance. It can save the hard disk space and also reduce the size of registry.