Volume compressor/limiter -- software?

[jmar]

Watching youtube on my HTPC is a pain because of the dramatic shift
in user specified volume levels.

I thought of throwing my hardware yamaha compressor in there but is
there any software that will do the same thing and WORK?

Just something to keep the audio levels constant coming out of windows media
player?

I'll pay for the software if it does the job.

THANKS