I need some way to control the audio output on my soundcard via software. Are there programs that will limit frequency ranges?

I don't know of any. The only software I know about that gives you detailed control over frequencies is playback software. For example, WinAmp has an equalizer, but it only works for music you play in WinAmp. It doesn't work for all the programs on your computer. The thing you're talking about would have to be done at the driver level or with a driver "shim" like Total Recorder. You might simply want to upgrade to a more advanced sound card with features that let you control this stuff.

ps- If I have a 10W amp feeding 15W speakers, is there such a device that I can use to boost the power level up in-between?

Yeah, a bigger amplifier. But the thing is, assuming these are speakers that aren't self-amplified, remember that that 15w rating is their maximum power handling. You don't HAVE to drive 15w speakers with a 15w amp, the 10w amp will do just fine. If all you want is louder, and turning up the volume doesn't work, then yeah, you need a bigger amp. Maybe you could buy an external equalizer while you're at it and kill two birds with one stone.

Now, these two questions in the same post make me wonder why you're asking in the first place. You want to limit frequency ranges, and you also want to know about the wattage of driving speakers. Are your existing speakers are distorting on heavy bass sounds and you want to fix that?