Moving the program does not guarantee a clean installation.
Try downloading and running the installer, using a Full installation. If you are told that there's already an installation at the specified location (normally the installer will default to the Upgrade option if this is the case), then pick a different location.
If you still have the same problem after this, take a look at what programs you have running, and the amount of memory each one is using, and compare that to the total amount of memory on your system.
The fact that you have to End Task mIRC means that the problem is with your computer, rather than a connection/network issue.