One of my favorite discount audio merchants, Audio Advisor, is closing out its stock of the very decent Boston Acoustics MM220 powered computer speakers.  While not as bass-rich as three-piece systems featuring a separate subwoofer, these slim towers do a decent job with low frequencies given their small size.  Audio Advisor has a YouTube video review with details.   If you do video conferencing or VoIP telephone calling from your PC (assuming you are not using a headset), or just like to listen to music while your work, the sound quality improvement should make it $30 well spent.