Summary: Mice pointer goes glitch/laggy while pressing keyboard at same time

My problem is: when I press any key on my keyboard my mouse pointer lags/glitches and I lose all precision. This makes specially hard to play with any ranged class, since its almost impossible to maintain precision while I'm using like SPACE to stay grounded and aim mobs.

My mouse is a X-luca team scorpion (optical) and sometimes I use a laser microsoft mouse (still getting the same glitchy response). Its important to note that my keyboard is a little old and has a PS/2 connector and have guessed that this could be the problem, but I'm not very inclined to this option, although I wish to hear your opnion before getting a new USB one.

That in fact is a minor issue for me, cuz I can play in a Mac machine with winxp (that doesn't respond with the same bug) and for the past one year I almost always change to Mac when I wanna play ranged. But I tryinglots of mice configuration and still the bug havent gone away, so im wondering if you guys would help me.

Ty in advance