The sound volume keeps changing itself

Yeah, that's the problem right there. I have tried using both a sound card and the onboard sound, always the same problem. It isn't the speekers since it's the same on another pair of speakers aswell as the ones I use right now.
Can it be a software problem? Most often after It has been on for awhile, regardless of the volume I have put it on, it settles on a setting that is a little to low and if I decide to change the volume again it starts freaking out.

Do you happen to have a keyboard with multimedia controls on it (ie. a volume control)? It could be conflicting with real keyboard commands. For example, on my laptop the volume control buttons actually map to other key combos (I forget which, but pressing the volume down button while playing Half Life on it causes me to crouch...).