Keyboard problem

2 weeks ago i upgraded my rig, everything was ok until i noticed the motherboard doesn't have any ps2 ports, i was able to get a ps2-to-usb connector, the only problem i have, i that, if you press any key while holding other key and you release any of those two, the key doesn't keep working, it's something like this:

You press W to run forward, then E to run diagonally, you release E and W stops working, so in short you can get to keep running, jumping, etc without having to press the key again, and, at least for me, it's pretty annoying, I've tried using a xbox 360 controlled along with xpadder, but as you can imagine it's not confortable at all.

I'm almost sure it's a hardware problem, but if someone knows how to fix this via software/system configuration i'd appreciate the help.

Keyboard model - A4tech G800
OS - Windows 7 Ultimate 64bits
Motherboard model - Intel DH77EB