Prevent software slow down

I wrote a program in C# WPF for a rapid response team. OS is Windows 7. They noticed that if the software left alone several hours and they wanted to use them it will takes to max. 30 sec to be the software responsive. We think that the OS swapping the program into the disk but I don't know how to figure out what is the real reason. It may help if we replace HDD by SSD but we would not like to do that first. Is there any way to prevent software to swap into disk? Or is it OS responsibility exclusively?

Thank you! Attila