How do I get rid of in-game delay?

I received a "new" computer from my dear friends. Although it's a lot faster than my old one, there seems to be an issue when I play games, both offline and online. I'll try to explain the issue as best as I can.

When browsing the internet or just my files there's no problem, but once I start up a game I get a delay from both my mouse and keyboard. The mouse-cursor acts as it should but once I give a "command" (e.g. walking, casting, etc.) the "game", as it were, doesn't act out the "command" until after a 0.5 second delay. The same applies to any keyboard "command" (eg. typing, casting, etc.). Though this may sound as virtually nothing it really slows down game play and split-second reflexes are virtually useless. The games themselves run fine: no lag, normal frames, normal ping, etc.

In short, there's an in-game delay between "command" (mouse and keyboard input) and "action" (in-game representation of input).

I have tried:

- Putting all games into the firewall exceptions

- Installing the latest DirectX (DX 11)

- Installing the appropriate graphics card driver(s)

- Putting my mouse and keyboard in different USB ports

Possibly relevant information:

- It's a Logitech wireless keyboard and a Trust wireless mouse. I've had both on my last computer and both gave no problems at all.

- GFX card is a GeForce 8400 GS. Nvidia drivers have been installed.

- The games I've tried/tested it with are Path of Exile (online), League of Legends (online) and Titan Quest: Immortal Throne (offline).

Any ideas are welcome. I've already tried asking multiple people within my vicinity but none seem to have any idea what could be wrong.